The first two vanish into the back room to play a game of darts, leaving the third to chat up the barmaid. After emptying a stein, he pulls a bit of ribbon from his pocket and entertains her with a trick whereby he ties the ribbon to the handle, twists the stein around two full turns, and then magically untwists the ribbon without moving the stein. He tells her how particles he studies have a property called "spin", and that particles whose spin is 1/2 actually behave like the stein with the ribbon tied to it: coming back where they started only after two, but not one, full turns. Feynman then leans closer, and conspiratorially whispers to her why (in language a barmaid can understand), just spin 1/2 particles act this way, and not other particles. What story does the Feynman tell the barmaid?