The artificial insemination man

Amy, a city girl, marries a farmer.

One morning, before he goes out to the fields, the farmer says
to her, "The artificial insemination man is coming to
impregnate one of our cows today. I drove a big nail into the
two-by-four over the cow's stall¹. You show him where it is."

The farmer leaves, and a while later, the artificial insemination
man arrives. Amy takes him down the rows of cows until she
sees the nail.

She says, "This is the one, right here."

The man says, "How do you know?"

Amy says, "By the nail over its stall."

The man says, "What's the nail for?"

Amy says, "I guess it's to hang your pants on."