Alice and Bill are in their sunset years in a nursing home. Both of their minds have started to slip away, as minds sometimes do at that age. So, one day, Alice is convinced that she's a differentiable operator. She runs around, screaming and pointing at people and saying "I differentiate you! I am d/dx!" Almost all of the other residents act as though they're in pain and move away when Alice yells "I differentiate you!"
Almost everyone, that is, except Bill. When Alice got to Bill, she yelled out "I differentiate you! I am d/dx!" ....Bill had no reply, except to pick up a newspaper and start reading.
Alice again screamed "I differentiate you!"
...Bill flipped to the Sports section.
Again, Alice screamed out "
I differentiate you! I am d/dx! You will be differentiated!"
To which Bill replied:
"Go ahead and differentiate me. I am e^x."