Theory: Objects expand when heated, and holes in objects expand as if they were made from the same material as the object.
Description: At room temperature, the ring will not fit over the ball. Show the class your propane torch, and ask them what you can do. If anyone suggests heating the ring, point out that this meanms they're suggesting that holes expand when heated; see how many people think that's true. Prove that holes do expand by heating the ring for about a minute in the torch flame, and then passing it over the ball.
As an alternative, the ball can be cooled in liquid nitrogen.
Setup time: 5 minutes
References: (available in the demo room)