Right in the beginning of Monsters Inc., the "training" monster eases up to the mechanical boy. There is a scene in which they show the monster's gleaming eyes under the bed. If you pay attention, there are several items under the bed INCLUDING a soccer ball. Now, I know I'm not mistaken, but when the mechanical boy screams, it startles the monster and he steps backwards and slips on the soccer ball, creating complete chaos. Unless the ball grew legs and walked up from under the bed, It's really not explainable.