A skeleton walks into a bar, and says "I want a beer and a mop."
This one's nice and short, so it's easy for you to remember and tell your English speaking friends in the pub tonight.
A skeleton is the whole collection of bones in the human body.
A mop is the thing you use to absorb the liquid that you have dropped on the floor, for example. You can actually say to mop something up. So of course, the skeleton will drink the beer and it will all end up on the floor. Not funny? Well, you don't appreciate my particular stupid sense of humour!