What's the only joke the clown told that made Batman laugh?

Batman: We should stop. We shouldn't do this forever. You know, one day, I will kill you, or you will kill me. Stop. I want to help you.

Joker: I really want to stop, really. But I can't stop because I'm crazy, you know?

Joker: I'll tell you a joke: A long time ago, there was a mental hospital. Two crazy people are good friends. One day, they were going to escape from prison, and they climbed onto the roof. One of them jumped over the roof and told the second one to jump over. The second one didn't dare to jump and said, "The roof is too high, I will fall to my death!" " The first one said, "I turn on the flashlight and you come along the beam!" " ! ""The second one looked at the first one and said, "I see. Turn off the flashlight when I'm halfway there!" "Isn't that funny? hahahaha ...

Clown: Ha ha ha ha ha ha ha. ...

Batman: ...

Clown: Ha ha ha ha ha ha ha. ...

Batman: Hum, haha. hahahaha ...