A mother and father take their 6-year old son to a nude beach.
As the boy walks along the sand, he notices that many of the women have boobs bigger than his mother's, so he goes back to ask her why.

She tells her son, "The bigger they are, the sillier the lady 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 things than his dad does.

She replies, "The bigger THEY are, the dumber the man is."

Again satisfied with her answer, the boy goes back to the ocean to play.

Shortly thereafter, the boy returns again, and promptly tells his mother:

"Daddy is talking to the silliest lady on the beach, and the longer he talks, the dumber he gets."