The Derivative Song:

You take a function of x and you call it y,
Take any x0 that you care to try,
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.

Source : ICI