"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."