Q: Why don’t people trust stairs?
A: Because they’re always up to something!

Explanation: Do you take the elevator or do you take the stairs?  Well, most of the places I go do not have elevators, but when they do, I usually take the stairs (unless the building is really tall and I have to go to an upper floor!).  There are lots of reasons to take the stairs.  Take the stairs is an idiom meaning walk up or down the stairs; it is similar to take the bus.

Up to something is also an idiom. If you are up to something, it means that you are planning something or involved in something, probably something bad.  A parent might say, “The kids are really quiet right now; I hope they’re not up to something.”  Up to something is not always bad.  I might say, “I knew my friends were up to something when everyone was too busy to for dinner on my birthday.”

This joke is funny because it plays with the idea of up to something.  Stairs literally go up to an upper floor, while people who are planning something are up to something and you might not trust them.

Here are some Looney Toon characters who are up to no good: