A farmer and his wife were lying in bed one evening; she was knitting and he was reading the latest issue of Farmer’s Weekly.
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 yes? Prove it.”
He frowned for a moment, then said, “Okay.”
He then got up and walked out, leaving his wife with a confused look on her face.
About a half an 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 was squealing, I couldn’t be sure.”