Look around you.  Wherever you are, you are surrounded by “stuff.”  But a good scientist doesn’t go around calling stuff, “stuff”!  That’s too simple.  Instead, a really good scientist calls all this stuff we are surrounded by, “matter”.   So what exactly is matter... besides being “stuff”?  Matter is anything that (1) takes up space and (2) has mass (or weight).

All matter on earth may be classified as either solids, liquids or gases.  These are known as the three states of  matter, and we will be spending some time exploring the properties (or characteristics) of each. 

As you begin your study of solids, liquids and gases, you’ll be doing a number of experiments that you may have done before.  You may be tempted to skip over them, but you really should try them again.  Why?  Because eventually, you’ll be learning how individual molecules of substances behave, and by taking time to closely observe how solids, liquids and gases behave in these familiar experiments, you’ll probably see some things you did not notice before.  Then, you’ll be better able to understand what is going on with the molecules of these substances when the time comes.

Gases are one state of matter.  Since all matter must have mass and take up space, if gases are matter, they must have must have mass and take up space.  Let’s see if we can show whether they do.

Materials Needed: Balloon.

Procedure:   Blow up the balloon, and tie it off.  Squeeze the balloon gently.

What To Look For: Notice how the balloon fills with air, and when you squeeze it, how it resists the pressure of your hands. Also, notice that when you squeeze the balloon in one place, it bulges in another.

What Happened: It is obvious from this experiment that air takes up space.  If it did not, the balloon would not expand when you put air into it by blowing it up.  This simple experiment shows that air meets at least one of the two tests for matter - it takes up space.   

We can show that air takes up space in another way as well.

Materials Needed: Sink or large container full of water; drinking glass or jar; sheet of paper.

Procedure:   Wad the sheet of paper and stuff it into the bottom of the container so that it won’t fall out when the container is turned upside down.  Turn the container upside down and push it down into the water until it is completely covered.  If possible, move the jar or glass so that you can see where the water is inside.  Remove the container from the water and observe the paper inside.

What Happened: The air inside the container would not allow the water to go inside and the paper remained dry, again showing that air takes up space.  If you observed the water carefully, you may have noticed that it was able to move up inside just a short distance.  The pressure exerted by the water compressed the air inside slightly.

More about that later!

Materials Needed: 2 jars, a sink filled with water.

Procedure:   Submerge one of the jars into the water and allow it to fill with water.  Keeping the jar under water, turn it upside down.  Next, turn the second jar upside down, and push it under the water.  Bring it under the first jar and tilt it up slightly to begin “pouring” air into the  first jar.

What Happened: You obviously realize by now that the air filled the second jar.  Since the air was lighter than the water, when the second jar was tilted, it allowed the air to escape and begin bubbling up toward the surface.  However, the air was trapped by the first jar, and the air forced the water out of that jar.  Again, you have shown that air takes up space.

OK, so after three simple experiments (some might even say "lame" experiments, you are willing to admit that air takes up space, but does it meet the second test for matter?  Does air have mass?  Let's find out.

(We’ll use the term, “mass” instead of weight because it is a little more accurate, but whenever we do, you can assume we’re talking about weight - at least here on Earth.  You may have learned from the Measuring Mass page that there is a difference between the two, but it only really matters where gravity has a stronger or weaker pull than here on Earth, such as on another planet, or out in space.)

Materials Needed: Two large balloons; homemade balance  or  yard or meter stick or a good school balance such as a triple beam balance; push pins or thumbtacks; pin.

Procedure:   Blow up both balloons and tie them off.  Use the meter or yard stick as the balance arm on your homemade balance. Fasten one balloon to each end of the balance arm with a push pin or thumbtack.  Adjust the balance so that it is level.  At this point, the masses on both sides of the balance are equal.  (NOTE:  If the balloons are big, you may need to position the balance on the corner of a table so that the balance arm can move freely.)

Now carefully pinch one of the balloons near the neck and stick a hole in the pinched area with the pin.  If you are careful, you can easily put a hole in the balloon without bursting it.  Watch what happens as the air leaves the balloon.

Or...

If you don't want to make the balance, you can just use a meter or yard stick suspended from a piece of string tied in the middle.  Once you have the ballons attached, balance the two sides so that the stick is level.  It might be a little more tricky to balance than when done with the homemade balance, but it can be done.  You just have to be a little patient.  When the stick is balanced, the total mass of each side is equal.  Carefully stick one of the balloons and observe what happens.

Or...

Use a single balloon - the larger the better - on the school scale.  Weigh it.  Then blow it up, tie it off, and weigh it again.  Is there any change?

What Happened: You could hear the air rushing out of the balloon you pricked, and as it did, the side of the balance that this balloon was on got lighter. You could tell this because that side of the balance rose above the other. Since the mass on that side decreased,  and since (almost) the only thing that was removed was the air inside the ballon, this shows that the air has some mass.

If you used a school balance or scale, depending on how large the balloon was, you probably saw that the inflated balloon was at least 1/10 of a gram heavier, and possibly considerable more.

One other thing that has to be considered when you blow up a balloon is that your breath has some moisture as well, and that adds at least a little bit to the weight.  However, you will still clearly see the change in weight if you are able to use perfectly dry air.  However, when you do any experiment, you should always look for things that you might affect your results.  Failing to do that has sometimes caused good scientists to misinterpret their results.

Going Further: If you can, get a football or basketball, and let the air out of it.  How heavy does it feel?  Now inflate it.  Can you tell a difference?  Although air, as well as other gases, are much lighter than solids or liquids, they do have some weight.  Assume that the balloon you deflated held about a liter of air.    A liter of air has a mass of a little less than a gram. That isn’t very much.  However, a one inch square column of air going from the ground to the edge of space weighs about 6.7 kg (14.7 pounds).  All that air does add up!

The air around you has much more of an effect on you than you might think, as these next series of experiments will show.

Materials Needed: Clear soda straw; glass of water.

Procedure:     Stick the straw down into the water.  Observe the water level inside the straw.  Next, remove the straw and place your thumb over the top of the straw.  While holding your thumb over the top of the straw, stick the straw down into the glass and note where the water level is inside the straw.  Leve the straw where it is and remove your thumb from the straw.  Observe the water level again.  Finally, place your thumb over the top of the straw while it is still in the water and lift it out.  What do you see?

What To Look For:   You should notice that the change in the water level is affected by whether the air can get in or out of the top of the straw.

What Happened: When you first placed the straw in the water, the air was pushed out of the top of the straw by the water below.  However, when you placed your thumb over the top, the air was not able to leave and it exerted pressure on the water to keep the water from filling the straw.  When you lifted your thumb, the air could escape from the top of the straw, and the straw filled with water.  Finally, when you replaced your thumb and lifted the straw from the water, the water remained inside the straw, since air could not take the water’s place through the top of the straw.

Materials Needed: Medicine dropper; glass of water.

Procedure: Place the tip of the medicine dropper under the water and squeeze the bulb.  What happens?  Next, release the bulb.  What happens now?

What Happened: This was so simple that you probably knew exactly what was going to happen before you even did it.  When you squeezed the bulb, you forced some air out of the dropper.  You saw the air bubble out and up to the surface. When you released the bulb water was sucked inside the dropper.  But as simple as this is, it is the principle on which pumps are based.

Materials Needed: Small can; small jar or bottle whose mouth is smaller than the bottom of the can; nail; hammer; modeling clay; water.

Procedure: Using the nail, punch a hole in the bottom of the can.  Roll out a thin strip of modeling clay just long enough to fit around the mouth of the jar.  Press the clay around the mouth and place the bottom of the can on top of the clay.  Press down enough to make a good seal between the jar and the can, but don’t push so hard that you cut completely through the clay.  It would be a good idea at this point to move the can and jar to a sink just in case there is a spill. 

Fill the can with water and observe what happens.  Next, tilt the can just enough to break the clay seal.  Now what happens?

What Happened: When you poured the water in the can, it was not able to flow down into the jar below because of the pressure being exerted on it by the air in the jar.  Because the air in the jar had no way to escape, the water in the can could not push it out of the way.  Once you broke the seal, air could then flow out from the jar, and water was then able to flow from the can into the jar.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Going Further:  Try doing this with a small plasitic container instead of can.  Start with a single nail hole. Make the hole a little larger and see whether water is still trapped in the top.  Repeat until water is able to get through and observe carefully.  What do you see?  Why do you think the water is now able to get through?

Here is yet another example of air pressure that you’ve probably seen before, but with an added twist.  As you do it, see how it is similar to the last experiment.

Materials Needed: Small jar (or glass); 2 pieces of cardboard big enough to cover the mouth of the jar; nail; toothpick; water.

Procedure: This is another one that is probably best done over the sink.  Fill the jar to the brim with water and place one of the pieces of cardboard over the mouth.  While holding the cardboard in place, turn the jar over.  Let go of the cardboard.  What happens?  Tap the corner of the cardboard.  Now what happens?

Next, take the other piece of cardboard and punch a small hole in the center with the nail.  Fill the jar to the top and place the piece of cardboard over the mouth as before.  Again, holding the cardboard in place, and with a finger over the hole, turn the jar upside down and let go of the cardboard.  What happens?  Remove your finger from the hole.  What happens now? Stick the toothpick through the hole and let it go.  What do you observe?

What Happened: In both cases, when you turned the jar over, the cardboard stayed in place.  Air pressure pushing up on the cardboard prevented the cardboard from falling off.  However, when you tapped the cardboard, you probably added just enough downward force to overcome the force of the air pressure, and the cardboard came off.  (You did do this over the sink, didn’t you?)  When you did this the second time, water could not flow out of the hole due to the air pressure.  The water was also prevented from running out by something called “surface tension”, and you’ll learn more about that on the Liquids pages.) By the way, the toothpick just adds a nice touch to keep your friends guessing.

Going Further: You’ve probably figured out already that there are limits to how big the hole in the cardboard (or the can in the previous experiment) can be and air pressure still prevent the water from flowing.  You may have also wondered how small or large an opening in a jar, glass, or maybe even a shallow pan, can be and this still work.  You may also want to see whether the size of the container makes any difference.  Perhaps you could design a science project to answer one or more of these questions.

If you just want to have some fun with your friends, get a flat piece of clear stiff plastic packaging material of the sort that many items are “blister” packed in.  Cut a flat piece just a little larger than the mouth of a glass or jar.  With a little practice, you should be able to hide it in your hand and “palm” it over the full container.  When you turn the container over, it will look like the water is being held inside the container with nothing underneath.  If you are careful, you may even be able to make a small hole for a toothpick, as in this experiment. 

You may recall that that a one inch square column of air going from the ground to the edge of space weighs about 6.7 kg (14.7 pounds).  This much weight exerts a significant amount of pressure, and it can lead to some pretty interesting results.

Materials Needed: Plastic soft drink bottle with cap (2 liter works very well); nail; water.

Procedure: Using the nail, punch a small hole in the soft drink bottle about 1/4 of the way from the bottom.  Do the same thing about 1/4 of the way from the top directly over the first hole.  Hold fingers over both holes and fill the bottle to the top with water. Place the cap on the bottle.

Now remove your finger from the top hole and observe what happens.  Place your finger back over the hole and remove your finger from the bottom hole.  What do you see now?  Finally, remove your fingers from both holes.  Can you explain what you observe based on the previous experiments you have done?

What Happened: This one is for you to figure out.

Going Further: Gather several plastic drink bottles of various sizes, and repeat this experiment except place the holes in different locations and see what happens.

In the previous experiments, you have seen several examples of the effects of air pressure.  In these next experiments, you will see that this air pressure can be quite strong.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Radish; smooth counter top; towel; smooth lightweight saucer.

Procedure: Cut the radish in half and hollow out the center of one of the halves.  Press the radish firmly down on the counter top while squeezing it slightly.  Now lift the radish.  What happens?

Place the saucer on a towel.  The towel will be used as a cushion for the saucer. Next, hollow out the other half of the radish, and press it firmly down onto the center of the saucer.  Carefully lift the radish.  You don’t have to lift it very far!  What happens?

What To Look For: You have just constructed two small suction cups.  If they don’t work like suction cups, make sure you made a nice even cut when you cut them in half.  If there is a gap around the bottom where air can get in, it won’t work.  If the radish is dry, you might want to moisten it using a little water.

What Happened: When you tried to lift the radish from the counter top, you had to exert a little force.  The surrounding air pressure prevented you from easily removing the radish.  The air pressure should have exerted enough force on the other half of the radish to allow you to lift and hold the saucer.

Going Further: Just how much weight can you support with a radish suction cup? Try picking up other heavier dishes.  Is there a limit?  When you try this, be sure to use a cushion under the dishes, and lift only an inch or so.  You don’t want to break anything!  Also, you might want to try other fruits or vegetables such as potatoes, beets or oranges.

Materials Needed: Suction cup dart from a toy gun; towel; saucer.

Procedure: Fold the towel to use as a cushion for the saucer. Stick the suction cup dart to the middle of the saucer and lift up an inch or so.

What To Look For: Is this suction cup stronger or weaker than the radish in the previous experiment?  Why?  How can you tell?

What Happened: This really depends on the type of suction cup dart you used, but whether it was stronger or weaker depends mostly on (1) whether the suction cup made a tight seal with the saucer and (2) how big it was.  The bigger around the suction cup is, the more surface area there is for air pressure to act on.  If this isn’t immediately clear to you, try the next experiment.

Materials Needed: Plumber’s friend; smooth wall or floor.

Procedure: Push the plumber’s friend firmly against the floor or wall and push the air out.  Now pull the plumber’s friend away from the floor or wall.

What Happened: It was obviously much harder to remove the plumber’s friend than it was to remove the suction cup dart.  The suction cup on the end of the plumber’s friend is much larger than the suction cup on the dart, and there is a much greater surface area for air pressure to affect.  It should not be surprising, then, that if there is more air pressure being exerted on the plumber’s friend, more force is going to be needed to remove it.

Remember how we said that a square inch column of air weighs about 14.7 pounds?  Well, that exact weight or pressure changes as the weather changes.  Lower pressure generally means wet weather, and higher pressure generally means fair weather.  As you go higher above sea level, the pressure also drops.  So perhaps a better way of saying this is that the average air pressure at sea level is about 14.7 pounds per square inch.  In these next two experiments, we will see how you can observe the day to day changes in air pressure.

We measure changes in air pressure using a device called a “barometer”.  This first barometer shows a change in air pressure by showing changes in the water level of water inside of a bottle.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Plastic or glass soft drink bottle; small bowl (a whipped topping container works great); two rubber bands; ruler (one with a millimeter scale is best); small foam coffee cup; water.

Procedure:   Carefully cut out the bottom of the cup to make a support for the bottle as shown.  The cup must hold the mouth of the bottle about 1/4 inch above the bottom of the bowl.  You will also need to make two small cuts in this support so that water will be able to move freely between the bottle and the bowl. 

Fill the bottle about 2/3 full with water, and fill the bowl with water so that the water level will be higher than the mouth of the bottle. Place the cup support over the top of the bottle, and, holding your hand over the mouth of the bottle, turn it over and place the mouth of the bottle under water.  Remove your hand from the bottle.  You should try to get the water level about half way up the side of the straight portion of the bottle.  If you need to let some water out, lift the mouth slightly above the water.  If you need to add water, remove the bottle and start over.  (This doesn’t have to be exact, so don’t worry too much about the level, just get it close.)  Make sure that the bottle is level and steady, and that water can move freely in and out of the bottle through the cuts on the bottom of the support.

When you have the water level where you want it, fasten the ruler to the bottle with a couple of rubber bands.  It doesn’t matter exactly where you place the scale on the ruler, since you are going to use the ruler only to observe the change in the water level. This experiment should be set up where you can leave it undisturbed for a few days.  You should also set this up in a room where the temperature stays about the same.

Observe where the water level is on the scale. Check the water level every day for several days.  Each time you check it, write down the date and time, the water level, and what the weather is like outside.

What To Look For: If you look carefully on the scale at the water level, you will see that the water seems to be drawn up slightly all around the edge of the bottle.  Because of this, the water level does not appear to be a sharp line, but is, instead, a thin band.  You should always measure from the bottom of this band. 

The water level should change as the air pressure changes, but you will have to look carefully to see the change, since it is usually very slight.

What Happened: As air pressure increased, more pressure was exerted on the surface of the water in the bowl.  This forced a little of the water inside the bottle, and the water level rose.  When the air pressure decreased, a little of the water inside of the bottle was forced out by the air inside, since the air pressure inside the bottle was now greater than that outside.  This increased pressure pushed some of the water out of the bottle.

Here is another way to observe changes in air pressure.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Wide mouth glass jar; rubber balloon; rubber band; scissors; clear tape; broom straw or thin soda straw; ruler; modeling clay.

Procedure:   Cut the rubber balloon into a thin sheet and stretch it over the mouth of the jar.  With the balloon stretched tightly over the jar, wrap the rubber band around the mouth a couple of times to make a tight seal.  Using a small piece of tape, tape one end of the straw to the middle of the balloon, and let the straw rest on the mouth of the jar as shown.  Use a small lump of modeling clay to make a support for the ruler, and place the ruler near the end of the straw.  As in the previous experiment, you should place this where it won’t be disturbed, and in a room where the temperature is fairly constant. Watch the straw from time to time for a few days to see what happens.  As in the previous experiment, write down the date and time, the level of the straw on your ruler scale, and what the weather is like outside.

After you have made and recorded your observations for a few days, try this: Hold your hands around the side of the jar for a few minutes.  Do you see any change?  Does this explain why you needed to set your barometer up where the temperature stays about the same?

What To Look For: The straw should rise and fall as the air pressure rises and falls.

What Happened: When you stretched the balloon over the jar and sealed the air inside, the pressure of the air outside and the air pressure inside were equal.  When the air pressure outside increased, it pressed down on the balloon until the air pressure inside the jar again equaled the outside.  This caused the balloon to cave inward and the straw to rise up.  When the air pressure outside dropped, the air in the jar pushed outward until the pressure was again equal, which caused the balloon to bulge outward and the straw to fall.

When you placed your hands around the side of the jar, your hands warmed the air inside.  This caused the air inside to expand, the balloon to bulge upward, and the straw to move downward.  This tells you that changes in temperature can affect the results of you simple barometer, and shows why you should keep it where the temperature is as constant as possible.

Going Further: If you were keeping good records in this experiment and the last, you may have noticed that the air pressure was generally lower when the weather was damp or stormy, and higher when the weather was fair.  In fact, this is why air pressure is such a good forecaster of weather.  If you can find a real barometer, you may even notice that it has words like “rain”, “snow” or “stormy” at the lower end of the scale, “change” or “unsettled” in the middle, and “fair” or “sunny” on the high end.

People have been using siphons to move liquids for a long time.  These very useful devices depend on air pressure and gravity to work.

Materials Needed: Drinking glass; two flexible drinking straws; tape (If you have a short piece of plastic tubing, you can substitute that for the straws and tape.); water; sink.

Procedure:   If you are using drinking straws, push one of the straws inside the other.  You may have to make a small slit in one of the straws to  make them fit together.  Tape the two straws together, being careful that the tape makes a tight seal.  Bend the straws at one of the flexes as shown.

Fill the glass with water, and place the short end of the bend into the water. Suck on the straw to fill it with water and quickly bend the long end down into the sink below the bottom of the glass.  What happens? 

Repeat the experiment, except this time, do not allow the long end of the straw to fall below the water level.  What happens now?

What To Look For: When you allow the filled straw to drop below the level of the water, you should see water begin to flow from the glass.  However, if you don’t hold the straw down lower than the surface of the water, the water in the straw will simply fall back into the glass. 

What Happened: When you dropped the straw below the water level in the glass, the water in the straw was pulled down by gravity.  This created a vacuum which drew the water from  the glass through the straw and down into the sink.  For this to happen, the water in the straw had to be below the level of the water in the glass in the first place.  Otherwise, the water would have drained back into the glass.

Going Further: If you can get some clear plastic tubing from a hardware store, you can experiment with a number of different siphon designs.  CAUTION!  When experimenting with siphons, be sure to only use clean water.  Since you are drawing the liquid with your mouth, you don’t want to swallow anything harmful!

The siphon you made in the last experiment has one serious problem.  You cannot use it to move liquids that are dirty or unsafe, because you must first get the liquid flowing by sucking up some of the liquid into a straw or tube.  The device you will make in the next experiment uses the same principle as the siphon, but it will allow you to siphon liquids without the risk of swallowing something unpleasant.  It can also be used to provide a source of water in your home lab.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body,  particularly your eyes.

Materials Needed: Plastic gallon milk jug with cap and silicon sealant; plastic tubing (1/4 inch outer diameter or smaller); clothespin; knife. (You can get silicon sealant and plastic tubing from most any good hardware store.)

Procedure: Cut one piece of tubing about 9 inches long.  Cut another piece of tubing about two and a half times the height of the jug. Use a sharp knife or drill to bore two holes into the cap for the tubes.  The holes should be just large enough for the tubing to fit through. Run the tubes through the cap and place a layer of silicon sealant around both tubes to make the cap air tight.  Allow this to dry thoroughly!

Fill the jug with water and place the cap on the mouth of the jug.  Blow into the jug with the short tube.  Water should begin to flow from the long tube.  If the longer tube is held below the bottom of the jug, water will continue to flow due to the siphon effect after you stop blowing.  To stop the flow, you can use the clothes pin as a clamp on the longer tube.  The longer tube will need to remain below the level of the bottom of the jug, or else you will need to blow into the short tube again to restart it.

You have now completed your wash bottle.  With this device, you will have a supply of water ready for your lab when you need it.

What Happened: When air is forced into the jug, pressure is increased inside, and the increased pressure forces water up the other tube.  When the water in the tube falls below the water level in the bottle, gravity takes over and the water is siphoned out of the jug.

Materials Needed: Plastic soft drink bottle with cap; water.

Procedure:   With the cap off the bottle, hold you hand above the mouth of the bottle and squeeze.  What do you feel?  Screw the cap on tightly and squeeze again.  What happens when you squeeze the bottle now?  Now, fill the bottle completely with water, replace the cap and squeeze again.  What do you feel now?

What Happened: When you squeezed the open bottle, you forced some of the air out of the mouth.  When you placed the cap on the bottle and squeezed again, there was no place for the air to go, but you were able to squeeze the bottle together.  In other words, you were able to compress (or squeeze together) the air inside the bottle.  However, when you filled the bottle with water and capped it, you could not squeeze the bottle very much at all because you could not compress the water inside.

Gases such as air may be compressed, but liquids such as water, may not.

In an earlier experiment, you saw that air takes up space inside of a glass or jar when the glass or jar is turned upside down and placed under water.  In this experiment, you will take that experiment one step further to see how water pressure affects the air.

Materials Needed:  Drinking glass or jar; large aquarium, sink or bathtub; water.

Procedure:   Fill the aquarium, sink or bathtub with water.  Turn the jar or glass upside down and lower it into the water, slowly pushing it to the bottom of the container.

What To Look For: Notice the water level at the bottom of the glass as you lower it to the bottom.

What Happened: As you lowered the glass into the water, the water rose a little way up into the glass.  As the glass was pushed deeper, the water level inside the glass got higher.   This happened because water exerts pressure on the air inside the glass.  This  pressure increases as the depth of the water increases.  Because pressure on the air inside the glass increases as the glass is pushed deeper, the air is compressed.

You feel this pressure as you swim under water.  In fact, the deeper you swim, the more you have to work against water pressure to stay at that depth.  Submarines, which can go many feet below the surface of the ocean, must to be built to withstand tremendous pressure.  If they were not, they would be crushed.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Materials Needed: Plastic soft drink bottle with cap; barrel from a plastic pen; pliers; tape; hammer; nail; scissors or small screwdriver; modeling clay; water.

Procedure: Find an old plastic ball point pen with a plastic barrel.  Have an adult to help you remove the pen from the barrel. (A pair of pliers may help.)  If the barrel has a hole in the middle, you will need to cover the hole with tape.

Punch a hole in the plastic cap using the hammer and nail.  Using scissors or a small screwdriver, make the hole just large enough for the barrel to fit through snugly.

Push the barrel of the pen through the hole until about 1/4 of the barrel is above the top of the cap.  Press modeling clay all around the barrel and cap top to make a tight seal.  Fill the bottle about half full of water and screw the cap onto the bottle.  Make sure the bottom of the barrel is under water.  If it isn’t add a little more water.

Hold the bottle and barrel firmly and blow hard into the bottle.  Take your mouth away and watch out!
 
What Happened: When you blew into the bottle, you compressed the air inside.  When you pulled your mouth away, the increased pressure inside forced water up and out of the barrel, and if you weren’t quick, into your face.  Water squirted out until the pressure on the inside of the bottle was equal to air pressure on the outside.

Going Further: Try this on your friends... but make sure they have a good sense of humor!

Just as air may do some very interesting things when compressed, it also holds a few surprises when its pressure is decreased.  The first experiments on the next Gases page will show what happens when you lower air pressure.

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Don't stop now.  There's much more to learn about gases.  Be sure to check out Gases - Part 2.