He looked up from the page and said to her, "Did you know that humans are the only species in which the female achieves an 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 an hour later, he returned all tired and sweaty and proclaimed,
Spoiler:
