A farmer and his wife were lying in bed one evening; she was knitting, and he was reading the latest issue of Animal Husbandry.

He looked up from the page and said to her, "Did you know that humans are the only species in which the female achieves orgasm?"

She looked at him wistfully, smiled, and replied, "Oh, yeah? Prove it."

He frowned for a moment, then said, "Okay."

He got up and walked out, leaving his wife with a confused look on her face.

About a half hour later, he returned all tired and sweaty and proclaimed, "Well, I'm sure the cow and sheep didn't, but the way that pig's always squealing, how can I tell?"