The man asks for another beer, and when it came he reached into the cardboard box and pulled out a small black mouse. He set this mouse on top of the piano and said "SING MOUSE". The frog began to play the piano and the mouse began to sing.
A guy at the bar who was watching all of this walks up to the man and asks to buy this little band that the man had. After a bit of negotiating, the drunk guy agrees to sell it to the man for $1000. The man put everything into the cardboard box and ran out the door before the drunk could change his mind.
The bartender had been watching all of this goings on and said to the drunk "You fool! You just sold that little critter band you had for $1000 and you could have made a fortune off of it!"
The drunk laughed and said "I'm not the fool, the guy who purchased it is. Do you really think I would have sold it if that mouse could really sing?"
The bartender says "What do you mean, I was here and listened to that mouse sing!"
"The joke is on you and the man who bought that little band", laughed the drunk. "That mouse can't sing. The frog is a ventriloquist!"