The bartender serves the drink and says, "That'll be four dollars."

The customer pulls out a twenty-dollar bill and hands it to the bartender.

"Sorry, sir," the bartender says, "but I can't accept that."

The man pulls out a ten-dollar bill and the bartender rejects his money again. "What's going on here?" the man asks.

Pointing to a neon sign, the bartender explains, "This is a Singles Bar."