A million grains make a heap. Remove one grain: surely it is still a heap. Keep going. When does the heap disappear?
One grain seems too small a change to turn a heap into no heap at all. Repeat that thought again and again, though, and you end up with a single grain. That is not a heap. The trouble is that every tiny step sounds reasonable, while the final conclusion sounds plainly false.
This is the sorites puzzle, named for an ancient heap of sand. It exposes the slipperiness of words such as “heap,” “bald,” or “tall”: ordinary language often gives us no sharp line between cases. Philosophers respond in different ways. Some accept borderline cases; others question the step-by-step rule or explain how context shifts the boundary. No one answer has become uncontested.