1. Show them the container full of unpopped popcorn.
2. Lay the metal ball on top of the un-popped popcorn and explain that the metal ball will turn into a ping pong ball.
3. Shake the container gently. The metal ball will sink to the bottom, and the ping pong ball will rise to the top, making it look like the metal ball has transformed.
Objects with a density lower than the fluid that they are submerged in will float and objects with a greater density will sink. In this set-up, the un-popped popcorn form a coarse fluid. The metal ball has the highest density, the ping-pong ball has the lowest density, and the un-popped popcorn have a density somewhere in between. The fluid begins to flow when the container is shaken from side to side. The high-density metal ball sinks, while the low density ping-pong ball bobs up to the surface. If shown as a magic trick, it appears that the metal ball turns into the ping-pong ball. But this is not a Magic channel but a science one.