The functions are sitting in a bar, chatting (how fast they go to zero at infinity etc.). Suddenly, one cries "Beware! Derivation is coming!" All immediately hide themselves under the tables, only the exponential sits calmly on the chair.

The derivation comes in, sees a function and says "Hey, you don't fear me?"

"No, I am e to x," says the exponential self-confidently.

"Well," replies the derivation, "but who says I differentiate along x?"

Source: http://www.workjoke.com/mathematicians-jokes.html