Every Friday after work, a mathematician goes down to the Ice Cream Parlor, sits in the second-to-last seat, turns to the last seat, which is empty, and asks a girl, who isn't there, if he can buy her some ice-cream.
The owner, who is used to the weird, local university types, always shrugs but keeps quiet. But when Valentine's Day arrives, and the mathematician makes a particularly heart wrenching plea into empty space, curiosity gets the better of him, and he says, "I apologize for my stupid questions, but surely you know there is NEVER a woman sitting in that last stool, man. Why do you persist in asking out empty space?"