How do you get down off an elephant?
You don't. You get down off a duck.

Here's a similar joke, quoting Groucho this time: It's nice out. They should leave it out.
The duck down gag was my favorite joke when I was in elementary school and it still represents a kind of humor I love. Parsing sentences in your brain requires remarkable mental agility. During the course of the sentence, you maintain a list of words and concepts and when the sentence ends, you rearrange them into subjects, predicates, and objects until they make sense.
Your brain takes some shortcuts to try to make the parsing easier. When it finds phrases with "strong binding" it collapses them into single concepts. Oddly enough, "down off an elephant" has strong binding. I say oddly because it’s not a phrase you use a lot in everyday speech. (I'm making an assumption about you here.) Nevertheless, your brain collapses the phrase into a single image of someone dismounting a large mammal.
This kind of humor, then, counts on your brain binding the phrase, realizing the error, expanding the nicely packaged concept back into individual words, and then reparsing the sentence with the new information. For some reason, this is an enjoyable process for the brain to do. We have a name for this. It's called "funny".

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