Econ 101 assumes marginal changes in a variable. Say an economist wants to measure the effect of a change in income on the quantity demanded to see if the good is a normal good. The model would change income and leave everything else constant. A constant variable might be the relative prices of all other goods. The economist in this case might build an Engel Curve.
The rate of change along the curve is the marginal amount. In calculus this is the derivative. If you want to find the rate of change for one variable while leaving the other variables constant, you use a partial derivative.
Would an economist have a bumper sticker that reads, "How's my deriving?"
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