"It's ..." a long hesitation, as he thinks of how to summarize it to someone who doesn't have a head for numbers. "Canst use calculate unknown quantities from known quantities; this is done by a mathematics called algebra, where letters stand proxy for the unknowns. By application of algebra to geometry, canst calculate quantities for shapes--their volume, and circumference, and area. Canst know by it how much fabric is needed to make a shirt, or how much water might fill a glass. But some shapes cannot readily be calculated with simple algebra: the curves of waves, or the arc of an object thrown, or the way the gravity of the sun and the Earth act upon other bodies in the heavens. Calculus permits such things to be calculated."