The joke goes, “I just lost my mood ring. I don’t even know I feel about it.”
My informant heard it from his best friend and likes it because it reminds of that relationship, which he said is one based on humor. Their dynamic is always fun and things like puns and non sequiturs form a big part of it because they are often absurd and so a departure from the stressful real world. This joke is absurd, too. It’s funny because it implies that without a ring that is supposed to tell you your mood, you don’t know what your mood is or what your feelings are. It plays on the fact that mood rings don’t actually work and to know our feelings, we can simply ask ourselves how we feel. The joke makes the reverse seem true.
I think jokes like that, where we assert something absurd or untrue, are funny especially to the current generation because they’re a very self-aware form of humor (fits in with postmodernism). They don’t sound like typical jokes of the past, which are often very transparently jokes, but instead are just statements that we only know to be jokes because they are so untrue or absurd.