Have you ever wondered what a heap really is? Well, I don’t think so. But, let’s look at this. Suppose you removed a grain of sand from a heap a sand, will it still remain a heap? Of course, it will. Now, you go on removing each grain of sand until you think you have removed that grain by which the heap stops to be a heap anymore. Oh, you say that is impossible. So how many grains does it actually take to make a heap?
This is a kind of paradox of vagueness and is commonly referred to as the Sorites paradox.
For more information check Sorites Paradox at SEP.