Joke

At one Mensa convention, several members at a local cafe noticed the shaker with an S on top, for salt, contained pepper and their pepper shaker, with a P on top, was full of salt. How could they swap the contents of the bottles without spilling anything and using only the implements at hand? Clearly, here was the marvellous Mensa mystery!
They presented ideas, debated them, and finally came up with what they felt was a brilliant solution involving a napkin, a straw, and an empty saucer. They called the blonde waitress over to dazzle her with their solution.
"Ma'am," they said, "we couldn't help but notice that the pepper shaker contains salt and the salt shaker contains..."
"Oh, sorry!" interrupted the blonde waitress. "Here," and she unscrewed the caps of both bottles and switched them.