The dreaded clay ball

Seeing as how this week is the last week of school I let one of my more well-behaved classes have a 'free-choice' day. As a reward for their great behavior and effort in art, the class got a chance to use model magic, pipe cleaners, feathers, and 'google'-eyes to create whatever they like. For the most part there were really creative things: monsters, 'green eggs and ham', piglets, chickens, coil pots, superhero's....and then there where the clay balls....Inevitably I always end up with a few students who after 40 minutes end up with a clay ball. I don't understand it, I told them that when it dries it won't be able to bounce but alas, despite my best efforts, and with the option of creating ANYTHING they want, they make that. I don't know I don't quite get it...whoever said that creative problem solving wasn't important where probably the same students who used clay to make spheres :(