 Similar difficulties arise with regard to modification, which is clearly a matter of degree. If I bring an attractive shell home from a walk on the beach and put it to use as a paperweight, does the transport count as modification? If not, would washing the shell before using it be enough? Or polishing it to bring out the color of its markings? A further complication is that many uses to which naturally occurring objects are put cause modifications. An unmodified stone used as a hammer soon acquires a spherical shape (Schick &amp; Toth 1993, 130 ff.). At what point did it first count as modified, since even the first strike would break off some fragments? Or do these use-induced modifications not count for satisfying the definition at all? If not, why not? The stone is certainly being used intentionally for a purpose, so the rest of the definition is satisfied.