Calculate comes from the Latin calculus, a small stone: the pebbles Romans pushed around a counting board. From there the word went two ways. One branch grew into the calculus of Newton and Leibniz. The other stayed with stones, so a doctor will still call a kidney stone a renal calculus, and a dentist calls the hard scale on your teeth calculus too. Underneath both is calx, limestone, which also gave English chalk and, much later, calcium. So when a teacher writes the word calculus on a blackboard, it is the same stone at both ends: the one in the word and the one doing the writing.