A bunch of functions are hanging out at a bar. There are trigonometric functions, logarithmic functions, polynomials, all different kinds of functions just hanging out and drinking. Suddenly the door flies open and a differential walks in. The functions are all terrified. Some dive behind the bar, some jump out of the windows, and others hide under their tables. Everybody is generally trying to get the hell away from this differential.
However, there's this one function that just sits calmly at the bar sipping his drink. The differential walks up to him and says "What is this? You should fear me like everyone else. Why aren't you running away?" The function shoots the differential a nasty look and replies, "I am e^x, bitch! You can't touch me." The differential just looks at him and says, "Who said I differentiate with respect to x?"