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Batman the Killing Joke is a graphic novel written by Alan Moore, drawn by Brian Bolland and published by DC comics in 1988.

The Joker tries to prove even the most logical people can go insane in one bad day and  he tries to provoke Batman by attempting to drive Commissioner Gordon insane thus creates a psychological battle.

Towards the end of the story The Joker tells Batman a joke:

"See, there were these two guys in a lunatic asylum...

And one night, one night they decide they don't like living in an asylum any more. They decide they're going to escape! So, like, they get up onto the roof, and there, just across this narrow gap, they see the rooftops of the town, stretching away in the moon light... stretching away to freedom. 

Now, the first guy, he jumps right across with no problem. But his friend, his friend didn't dare make the leap. Y'see... Y'see, he's afraid of falling. 

So then, the first guy has an idea... He says "Hey! I have my flashlight with me! I'll shine it across the gap between the buildings. You can walk along the beam and join me!" B-but the second guy just shakes his head. He suh-says... He says "Wh-what do you think I am? Crazy? You'd turn it off when I was half way across!"