Babies can also do rudimentary math with objects. The demonstration is simple. Show a baby an empty stage. Raise a screen in the middle of the stage. Put a Mickey Mouse doll behind the screen. Then put another Mickey Mouse doll behind the screen. Now drop the screen. Adults expect two dolls, and so do five-month-olds; if the screen drops to reveal one or three dolls, the babies look longer than they do if the screen drops to reveal two.