Aids: a stick or a ruler, a big piece of newspaper Lay a ruler on a table.
Hit the ruler. The ruler flies away without breaking. Again, lay the ruler on the table and put a piece of newspaper on it so that there is no air under the newspaper. Flatten the piece of newspaper carefully. Quickly hit the ruler again. The ruler is broken.
The air above the piece of newspaper pushes it down with a big force. The ruler isn´t strong enough to lift the piece of paper. When you hit the ruler slowly, there is enough time for some air to flow under the piece of paper and to balance the air pressure under and above the paper.