Brass ball doesn't fit through brass ring until ring is heated.
What it shows:
Most solids (see Comments) expand when heated due to increased atomic and lattice vibrations. In this demo, a brass ring expands when heated to let a previously too small a ball pass cleanly through.
How it works:
The apparatus consists of a brass ring on a handle (figure 1), attached by a chain to a brass ball. Demonstrate that the ball is too large to pass through the ring, then heat the ring over a blue Bunsen flame for about a minute. The ball can now be lowered by the chain straight through the hole. Magic!
figure 1. brass ball and ring
Setting it up:
As well as the Bunsen and its gas supply, a pair of insulating gloves just to be on the safe side.
An inexpensive version of this demo is available from Cenco. 1 A good companion demo would be the Giant Spring Crystal to show what's happening on a microscopic scale. Rubber is a solid that contracts upon heating. A large rubber band, held taught by a suspended mass, will contract when heated with a heat gun; the mass is seen to rise. A demo presently on the drawing board!
1 CENCO 77450