The nudist colony

A married couple go to a nudist colony with their four-year-old son. The son asks, "Dad,
why do some men have big knobs and others little ones?"
The father, not wishing to get into a discussion, replies, "The Men with the big knobs are
smart and the Men with the little ones are dumb."
The little boy accepts the explanation and doesn’t ask any more questions.
A little while later the father is looking for his wife and can’t find her.
So he asks his son, “Do you know where mummy is?”
“Sure” replies the boy, "She's behind the bushes with this really dumb guy who is
getting smarter by the minute!"