Interesting Philosophy Bites on vagueness or the Sorites paradox. The notion is simple: when does a man become bald? Is there a point where the man having one hair means he is not bald and when he loses that one extra hair, he then can be classified as 'bald'? Well, of course, the concept "baldness" is vague: there is no quantifiable amount of hair or a sharp line between being bald and not being bald. We just apply it to cases where it seems to fit.
This can put some philosophers in fits because one stated goal of semantics is to find the exact, specified conditions that make up the truth conditions of the application of a concept. "is a firefighter" is an easy one. "is bald." or "is a heap of sand.", not so much.