Get a long copper tube (3 feet is good) and a strong magnet that's an easy fit. Ask them to guess how long different objects will take to fall through the tube

Get a long copper tube (3 feet is good) and a strong magnet that's an easy fit. Ask them to guess how long different objects will take to fall through the tube. Have them time them. Try a glass marble, steel marble. Then blow their minds with the magnet.