Two different experiments to show air pressure at work.
1) Newspaper, 'ACE' hardware yardstick (1/8" thick), a flat table. 2) Kite string, plastic straws, balloons, cellophane or masking tape.
First we'll show that there is air pressure pushing on us, from every direction
while we're on this Earth. Place a thin yardstick on a flat table with a little
less than half of it hanging off of the edge of the table (see the drawing below).
Place a sheet of newspaper over the yardstick flat against the table (have as little
air as possible under the paper) so that the fold line of the newspaper is at
the yardstick. Quickly strike the end of the yardstick hanging off the edge of
the table. If you strike it quick enough, the yardstick will break near the table edge.
Now, we're going to make a balloon 'rocket' that shoots along a kite string. Cut a plastic straw in half and tie a length of string (at least 20 feet long is more fun) between two chairs or something. Before you tie the second knot in
the string, slip the straw on to the string. Try to get the string fairly tight (the two chairs work well because you can pull the chairs apart to get the string tight). Blow up a balloon, but don't tie off the end and tape it to the straw so that it resembles the drawing below. Let go of the balloon and the 'rocket' should shoot along the string (very quickly) towards the other chair. Try different kind of balloons!
The Earth is covered in a layer of air that is nearly 80 miles thick and
at sea level (the bottom) exerts or 'pushes' almost 15 pounds of pressure per
square inch. That means that a full sheet of newspaper laid out flat has nearly
9,300 pounds of air above it. When you break the yardstick above, you are able
to break it because of that 'heavy' air pushing down on the paper while you quickly
strike the yardstick. Initially, the table is pushing back on the paper, and if
you move the yardstick quick enough, other air around the edges of the paper can't
get under the paper fast enough, so you are trying to lift that 9,300 pounds with
the yardstick! Some air gets under the paper, but not enough, so the yardstick breaks.
The 'Balloon Rocket' illustrates a way to use air pressure to move something. When you blow up a balloon you can 'see' air pressure as the balloon increases in size and the experiment shows exactly what happens when that pressure is released!
Not much mess, but you need to be careful not to use too thick of a yardstick, it should be about 1/8 of an inch thick, but not more. Also, don't have a friend stand right above the yardstick when striking it as the yardstick or a piece of it could hit them in the face.
That there is this tremendous pressure of the weight of the air pushing on us every moment of every day. The only reason we don't get crushed is that our bodies push back at the air with the same pressure.