Amy, a blonde city girl, marries a farmer. One morning, on his way out to the fields, the farmer says to Amy, "The artificial insemination man is coming over to impregnate one of our cows today. I drove a nail into the two-by-four just above the cow's stall in the barn. You show him where the cow is when he gets here, okay?" So the farmer leaves for the fields.
After a while, the artificial insemination man arrives and knocks on the front door. Amy takes him down the barn. They walk along long row of cows and when she sees the nail, she tells him, "This is the one. This one right here."
Terribly impressed by what he seemed to think just might be another ditzy blonde, the man asks, "How did you know this is the cow to be bred?"
"That's simple. By the nail over its stall," Amy explains. Then the man asks, "What's the nail for?"
"I guess it's to hang your pants on," she tells him as she walks away.