Clinging Cardboard

You will need: 2″ square of lightweight cardboard, small nail, and wooden or plastic thread spool.

Push nail through middle of cardboard. Then set cardboard on top of spool, with nail extending downward inside so that cardboard is flat against the end of spool. Now hold nail and cardboard in place and blow through it.

As soon as you start blowing, let go of cardboard. The cardboard does not blow away from the spool! As long as you keep blowing, the spool can be turned toward the floor and the cardboard will stay in place. Why does this happen?

By blowing, you make a current of moving air come out in a thin stream between the cardboard and spool. The pressure in this moving air is less than the pressure of the outside air. Consequently, the harder you blow, the more the outside air pushes against the cardboard to hold it in place.