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Mary, a young blonde city girl, marries an older New Zealand dairy farmer.

One morning, on his way out to check on the cows, old farmer John says
to Mary, "The artificial insemination man is coming over to impregnate one
of our cows today. I drove a nail into the 2 by 4 just above the cow's stall
in the barn. You show him where the cow is when he gets here, OK?"

Mary nods in agreement and the farmer leaves for the fields.

After a while, the artificial insemination man arrives and knocks on the front
door. Mary takes him down to the barn.

They walk along the row of cows and, when she sees the nail, she tells him:
"This is the one... right here."

Terribly impressed by what he seemed to think just might be another ditzy
young blonde gal, the man asks, "Tell me, Lady, how did you know this one
in particular is the cow to be bred?"

"That's simple. By the nail over its stall," Mary explains very confidently.

Then the man asks, "Well then, what's the nail for?"

She turns and walks away, and with complete confidence, says, "Well, I would
guess it's to hang your trousers on."