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Air is invisible, and it can push things around. Air, like everything else in the universe that you can touch, is made up of molecules, tiny pieces of stuff. And air molecules are always moving — bouncing around and bumping into things. When air molecules bump into things, we say the air has pressure. Try this:

What You Need

  1. regular sheet of paper, the kind you might use in a printer or copy machine

What You Do

  1. Put the paper on a table so that the long sides go left to right. Bring the short sides in so they meet in the middle and crease them.
  2. Place the folded paper near the edge of a table so that it looks like an upside down U.
  3. Put your mouth down even with the table and blow under the paper. The paper doesn’t fly up, it goes flat!

What's Happening?

There is air pressure all around us pushing in all directions at once. The pressure is even up and down and side to side, so the paper just sits there. But when you get air molecules moving with your breath, the molecules push more in the direction they’re going, and not so much up or down. Meanwhile, the air molecules above the paper are still pushing down as hard as ever. Since there’s more air pressure pushing down above the paper then there is underneath pushing up, the paper goes flat. It’s all done with moving molecules.

Bill Nye