A mother and father take their 6 year old to a nude beach. As the boy walked along the beach, he noticed that many of the ladies had boobs bigger than his mother's so he asked his mother about the size differences. 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 penises than his dad does and he wondered why. His mother again replied, "the bigger they are the dumber the person is."
Satisfied with his answer, the boy returned to the ocean to play. Shortly after, the boy returned again. He promptly told his mother, "Daddy is talking to the dumbest girl on the beach and the longer he talks, the dumber he gets."