A classical cash register is not the same like the thing we call computer, but that's not my point. My point was that "a computer is a thing that computes while a calculator is a thing that calculates" does not sound correct, the argument is basically the wrong way around. In my opinion, the verbs "to compute" and "to calculate" are synonyms. We only recently started to distinguish between a computer and a calculator, probably being a bit fuzzy and sloppy regarding the term "computer" (as natural languages are). But that doesn't mean that we can transfer that implicit (and maybe altered) meaning back to the verb "to compute".