One day, at the bar...

An old man walks into a bar and orders a beer. The bartender pours one from the tap, sets it down in front of him and says, "Three dollars, please". 

The old man stands up, goes to the right end of the bar and sets down a dollar. Then he walks to the left end of the bar and sets down another dollar. He gets back to the center of the bar and puts the final dollar in front of his beer. The bartender is quite angry but he can't refuse the money. The old man drinks his beer and walks away. 

The next day the old man shows back up, orders a beer and pays in the exact same manner. The bartender is getting more and more angry. For a week, the old man keeps coming and paying in this odd way. 

Finally, the old man notices that he's only got a five-dollar bill. He gives it to the bartender, who is finally ready to take his revenge. He pours the old man a beer and sets it in front of him, goes to the left end of the bar and puts a dollar down, and then to the right end of the bar and puts the second dollar down. He goes back to the old man with a big smile on his face.

The old man doesn't move, drinks his beer, then takes out his wallet. He pulls out a dollar, puts it in front of him and says, "I'll have another beer, please".