Latin quod erat demonstrandum, originating from the Greek analogous hóper édei deîxai (ὅπερ ἔδει δεῖξαι), meaning "which had to be demonstrated". The phrase is traditionally placed in its abbreviated form at the end of a philosophical argument.
You have to read for education. The paradox is in the undefined number required to make an armory or a heap and the induction of remove one and is it still an armory? Remove two, is it still an armory? Remove all but ten, is it still an armory? Remove the next to the last, is it still an armory? That leads easily to an (mathematical) infinite sum and proof by reductio ad absurdum.