I was in a bar in San Francisco one day and there was a dancer on stage. Since I was the only patron in the bar the bartender talked to me.

He told me that he employed twin dancers who were working their way through college. One was named Shelly and the other Jan. He said that one day Shelly would be dancing and the next day Jan would be dancing.

I asked him which one was dancing that day.

He looked up and said, "It must be Shelly 'cause Jan don't shake like that."