A derivative is walking down the street and sees a hot function.
So he grabs her, takes her into the bushes and differentiates her once.
Little later he sees another function and differentiates her twice.
He goes on an sees this super hot function. really sexy.
So he grabs her and differentiates her once and nothing, differentiates her second time, still nothing.
After 10 unsuccessfull differentiations he gets tired and asks the function:
"How come i can't differentiate you???!???"
And the function answers:
"HA!! That because i'm ex !!!"
and beats the crap out the derivative.