A farmer and his wife were lying in bed one evening. She was
knitting, he was reading the latest issue of Animal Husbandry.
He looks up from the page and says to her, "Did you know that
humans are the only species in which the female achieves orgasm?"
She looks at him wistfully, smiles, and replies, "Oh yeah? Prove
it."
He frowns for a moment, then says, "O.K." He then gets up and
walks out, leaving his wife with a confused look on her face.
About a half an hour later he returns all tired and sweaty and
proclaims, "Well, I'm sure the cow and sheep didn't, but the way
that pig's always squealing, how can I tell?"