A skeleton walks into a bar...

A skeleton walks into a bar and says, "Give me a beer and a mop."

The humor in this joke centers around the unstated idea that the skeleton is going to drink the beer, have no stomach in which to hold the beer, and then the beer will continue to fall down until it gets all over the floor. It will then need the mop to clean up after itself.

Figuring this out is the funny part of the joke. Additionally, why would a skeleton want a beer if he can't even properly drink it? This is an absurd situation.