A duck walks into a bar and orders a martini
A duck walks into a bar and orders a martini. The bartender stares, but mixes the drink, and the duck downs it and orders another. "Hey, buddy, you haven't paid for the first one! Why should I make you another?" "It's OK, make me a second martini," said the duck. "Just put it on my bill."
This is really a bad joke. The humor centers around a pun. "Put it on my bill" is a phrase that means "add it to the amount of money that I already owe you", but in this case (since a duck has a body part called a bill) it could also mean that the duck wants the bartender to place the martini on the top of the ducks bill body part.
Another, better version of this joke involves chapstick (which begs the question "why would a duck need chapstick anyway?")