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, "Okay."  He gets up and walks out,
leaving his wife with a confused look on her face.  About 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 keeps squealing, I
can't tell."