I'm going to make a joke about stealing from her
Why are you going to joke about stealing?
I was really pissed with her.
Joking around is really fun. Stealing is no joke, though. Never joke about something that is serious, if someone believes you and reports you as a thief, it might land you in jail.
Why can't I joke about something serious? I'm not going to do it for real?
People might take your joke seriously and you might get in trouble. You shouldn't joke about it.