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 nail into the two-by-four above the cow's
stall. You show him where it is, okay?"

So the farmer leaves for the fields, and a while later,
the artificial insemination man arrives.
Amy takes him down the long row of cows until she
sees the nail, and tells him, "This is the one. This one
right here!"

Terribly impressed, the man asks, "How did you know
this is the cow to be bred?"