Tornado-in-a-bottle

Materials:

Two plastic soda bottles with plastic caps

Clear "crazy glue"

Duct tape

Food coloring

Glue the flat ends of two bottle caps together. Let the glue dry overnight. Wrap the duct tape neatly around the outside of the double cap.

After the glue has dried, have an adult use a drill with a wide bit to make a roughly half-inch hole through the center of the caps. The hole should be just about as wide as possible.

Fill one of the bottles about three-quarters full with water. Add 10 drops of food coloring.

Screw the double cap on the bottle with water in it, and then screw the empty bottle on top.

Turn the contraption upside-down, give it a swirl, and see what happens! Gravity pulls the water in the top bottle down into the empty one. But the bottom bottle isn't really empty. It's filled with air, which gets pushed into the upper bottle as water rushes in. The air "punches" a hole in the water coming down, so it has clear passage into the top bottle. The water coming down swirls around the hole, making a whirlpool.
Tornadoes work the same way. When cool air flows into an area, trapping hot air beneath it, the hot air punches a hole through the cool air. The hot air then can rise up through the hole to form a tornado.

Nye Labs, http://nyelabs.kcts.org/homedemos/demo12.html