A tool is an object used to extend the ability of an individual to modify features of the surrounding environment. Although many animals use simple tools, only human beings, whose use of stone tools dates back hundreds of millennia, use tools to make other tools. The set of tools required to perform different tasks that are part of the same activity is called gear or equipment. While one may apply the term tool loosely to many things that are means to an end (e.g., a fork), strictly speaking an object is a tool only if, besides being constructed to be held, it is also made of a material that allows its user to apply to it various degrees of force. If repeated use wears part of the tool down (like a knife blade), it may be possible to restore it; if it wears the tool out or breaks it, the tool must be replaced. Thus tool falls under the taxonomic category implement, and is on the same taxonomic rank as instrument, utensil, device, or ware.