In the elevator in my apartment building this morning, I noticed something fairly sad.

There was, and always has been, a sign reading: "In case of fire - use stairways - do not use elevators."

Some clever sneak had written in red marker, "USE WATER" next to "Do not use elevators."

Now, apart from the fact that this is a tired old joke that was only moderately funny the first hundred times people did it, it didn't even work here.

See, this joke only works when the sign simply reads, "In case of fire, do not use elevators." This sets up the expectation that the sign is referring to what mode of transportation one is using, but leaves it ambiguous enough that the jokester can twist the expectation by implying that it's referring to what one uses to put out the fire. In the case of this sign, however, there was no ambiguity, and thus no humor. Plus, the sign already said to "use" something, so the whole rhythm of the joke is thrown off.

Why did this person think this would be funny?

I think our school system should teach, as a basic part of its curriculum, how to be funny. Or at least, how not to be completely unfunny.