I do not understand this joke, please help me

http://www.manythings.org/jokes/9965.html

A: "Excuse me. Do you know the way to the zoo?"
B: "No, I'm sorry I don't."
A: "Well, it's two blocks this way, then one block to the left."
I reckon Mr. A is suggesting that Mr. B should be kept captive in a zoo (probably because of the way he looks/behaves) and providing him with some pieces of information about the location of the zoo, so that Mr. B can get to it.

- YoHf [8]
The joke works on the idea that normally you ask directions to a place because you don't know how to get there yourself. This is funny as it reverses that idea - A has gone up and told a complete stranger, who doesn't care where the zoo is, the directions - the complete opposite of the real situation.
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I agree with Nona's explaination, that's the whole idea of the joke.
On the other hand YoHF's version is more ' imaginative' ....Emotion: big smile
I just remembered another one like that:

A couple was waliking along the dark street - Suddenly a group of youngsters surrounded them,

-Have you got a tin-opener?
- No, - said they
- Then stay here! Or you will suffer! (and the youngters ran away)
The couple stood scared for a quarter of an hour - then the boys appeared, gave them a tin-opener and said,
-Now you may go!
I understood it as it was explained by nona de brit. But I like the YoHf's explanation as well. Emotion: smile
It's funny.
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Yeah, I heard that joke before but in a different version..

A: Hey, you have a pen?

B: Oh, sorry I don't.

A: Well, I have one!