This won first place for the age 5 - 9 division at the Iowa State Fair:
Once upon a time, there were two mice, In and Out. They lived in a hole that was too small for both of them. When In was in, Out was out, and when Out was in, In was out. One day, Out came home and realized that In had died.
'How did Out know?'
We don't know. How?
'Instinct.'