Q. Why don’t skeletons fight each other?
A. They don’t have the guts.
Explanation: To “have guts” is to be brave. If you “don’t have the guts” to do something it means you are scared to do it. “Guts” are your innards, your internal organs (heart, lung, liver, …). This joke plays on the idea that skeletons don’t have guts (they are nothing but bones so they have no internal organs), or are they scared. The double meaning of the word “guts” makes this joke funny.