Here's a neat way to show how heavy air is when it presses down on our bodies, and you get to break things too...

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As we said in our other demonstration, an empty paper cup is actually full, full of air.

Air is made up of particles called molecules. A whole sky of air means we have a whole sky of molecules squashing down on us. You don't believe that we are being squashed right now?

Try this...

Place a long ruler, or similar wood, on the edge of a table so about half of it is off the edge. Take a double sheet of newspaper and fold it up into A4 (refill) size. Lay this on the part of the ruler that is on the table. With a Karate chop type action, hit the half of the ruler that is hanging off the table to try to flip the paper off the ruler. Was it easy? (Be careful, you might hurt yourself or break something around you!) Open the sheet out again and place it flat so it covers the part of the ruler that is resting on the table. PREDICT: Ask your students "What will happen this time? Will the ruler be as easy to flip off the table?" OBSERVE: Try the karate chop again...and again but harder... OK, really hard....oops, it broke! (Be careful!) EXPLAIN: What does this show us? (seniors might like to consider the forces involved)

By using the same sheet of newspaper, but folding it to different sizes, we have shown that it is not the weight of the paper that matters here. It is the size of the paper or its surface area. The more surface area, the more air is above it, pushing on it. The more air pushing on the paper, the more forcefully the ruler is pushed down onto the table!

When air pushes against something it is called pressure.

Air pressure is all around us as we live under a "sea of air" - a bit like a fish surrounded by a sea of water. The air presses on us from all side, but we are so used to it we don't feel it. Every part of our body is pushing back (each cell is like a balloon) so we don't get squashed flat.

If we travel in an plane, the higher up in the sky (atmosphere) we go, the less air pressure there is. If we go into space where there is no sea of air (no atmosphere) our bodies (our cells) still push outward.

This is why astronauts have to wear a space suit, not only for air to breath, but to contain air at the correct pressure to push back against their bodies.

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