Imagine you have a heap of sand. Now, if you remove a single grain of sand from a heap, it's still a heap, right? But if you remove every grain of sand, then eventually, you're left without any sand, which is definitely not a heap. So somewhere along the way, when removing individual grains of sand, the heap stopped being a heap. But where, exactly, did that happen?
(1) more than one vertical layer (the requirement for it to be a pile), and
(2) each grain not directly in contact with the ground being above a point on the ground within the triangle defined the points on the ground directly underneath three lower grains (distinguishing a heap from other piles).
(Of course, you may have a different definition of a heap, but if once you make concrete what you mean by a "heap", the problem is resolved. If you view heap as a fuzzy concept, then there is a continuous-valued fuzzy membership function, and there is no crisp point at which the collection stops, or starts, being a heap.)
Imagine you have a boat. Now take a piece off of it. Is it still a boat? How far can you go until it no longer holds the distinction of being a boat?
Yes, I know that submarines are colloquially known as (and are in fact) boats.