A farmer in Arkansas and his wife were lying in bed one evening. She was knitting, and he was reading the latest issue of Animal Husbandry. Suddenly, he looks up from the page and says to her, "Honey, did you know that humans are the only species where the female achieves orgasm?" She looks at him wistfully, smiles, and replies, "Oh yeah? Prove it."
He frowns for a moment, then agrees. He then gets out of bed and leaves the bedroom, leaving his wife with a confused look on her face. He returns a half hour later, all tired and sweaty, and says, "Well, the cow and sheep didn't achieve orgasm, but the way that pig squealed... it's hard to tell."