Song by Tom Lehrer to the tune "There'll be Some Changes Made". Published in American Mathematical Monthly, 81 (1974). Reproduced here with Tom Lehrer's written permission (see this node).

You take a function of x and you call it y,
Take any x-nought that you care to try,
You make a little change and call it delta x,
The corresponding change in y is what you find nex',
And then you take the quotient and now carefully
Send delta x to zero, and I think you'll see
That what the limit gives us, if our work all checks,
Is what we call dy/dx,
It's just dy/dx.