Sorites paradox

Sorites paradox (also called Paradox of the heap) is a paradox that arises when unclear terms are used. The name of the paradox is related to the Greek word for "heap" ('soros'). There are different versions, but a typical one involves a heap of sand. The heap is made of many grains of sand. So if one grain is removed, the structure will still be a heap. This process can be repeated, until only one grain of sand is left. Since the heap never changed to a non-heap, this grain of sand is still a heap.
The paradox works the other way as well. If one begins with one grain of sand and adds another, and then another; when do the grains of sand become a heap?