Oh, but it gets even better with Silly Putty... Silly putty is composed of a non-newtonian liquid which have awesome properties.

Alternate test:
Drop a metal ball onto flattened Silly Putty from various heights. At some height the force of impact will compel the Silly Putty into its elastic behavior and the ball will bounce. This shows that it is a question of the material properties reaction to force and not strictly a shape question.

He may also enjoy making a non-newtonian liquid using just corn starch and cold water. Stir a lot of cornstarch into the water until when the spoon moves too quickly it gets stuck. If he taps the surface with his finger at various speeds he'll get the transition effect. Fun but not as elastic as Silly Putty.