Say you have a heap of sand from which you remove one grain at a time. At what point does it cease to be a "heap"?
Or say you have a series of paint swatches that gradually shade from red to orange in increments too small for the observer to perceive. Drawing the boundary between any two swatches is impossible, and yet clearly some are red and some are orange.
The point is that while "heap" and "red" and "orange" do have useful definitions as terms of natural language, they're too vague to be useful in logic. Bertrand Russell argued that this is true of all natural language, even language specifically intended to represent logic.