The headmistress of a girls' school asked a male friend who was an author to give a talk to the pupils about sex. After much persuasion, the man agreed but was too embarrassed to tell his wife. So he told her that he was addressing the school on sailing and wrote an appropriate entry in his diary for that day.
The day after the talk, the headmistress met the wife in the street. "Your husband was wonderful yesterday, so illuminating. I know my girls learned a lot from him."
"I can't think how," said the wife. "He's only tried it twice. The first time he was sick and the second time he lost his hat."