Pianist's Monkey

A man walks into a bar and orders a beer. He sips it and sets it down. A monkey swings across the bar and pisses in the pint. 

The man asks the barman who owns the monkey. The barman replies, "The piano player". The man walks over to the piano player and says "Do you know your monkey pissed in my beer?" 

The pianist replies, "No, but if you hum it I'll play it."