"What is entropy? ...a measurable physical quantity just like the length ...temperature ...the heat of fusion ...or the specific heat of any given substance. At ... ...the entropy of any substance is zero. When you bring the substance into any other state by slow, reversible little steps ...the entropy increases by an amount computed by dividing every little portion of heat you had to supply ...by the absolute temperature at which it was supplied ...and by summing up all these small contributions."