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 and said, "OK," then got up and walked out, leaving his wife with a confused look on her face.

About half an hour later he returned all tired and sweaty. "Well, the cow and the sheep definitely didn't have one. But the way that pig squealed, it's hard to tell!"