dumb and dumber
A mother and father took their 6 year old son to a nude beach. As the boy walked along the beach, he noticed that some of the ladies had boobs bigger than his mother's, and asked her why.
She told her son, "The bigger they are the dumber the person is."
The boy, pleased with the answer, goes to play in the ocean but returns to tell his mother that many of the men have larger willies than his dad. His mother replied, "The bigger they are the dumber the person is."
Again satisfied with this answer, the boy returns to the ocean to play.
Shortly after, the boy returned again. He promptly tells his mother, "Daddy is talking to the dumbest girl on the beach and the longer he talks, the dumber he gets."