An old one: Rotarian is asked to give a talk on sexual variations to his colleagues, but tells his wife he will be talking about gliding.
Next day she meets another rotarian who says "Your husband's talk was fantastic - I had no idea he had so many experiences in such a variety of positions".
She says " That's funny; as far as I know, he only tried it twice. The first time his hat blew off, and the second time made him sick!"