A tool or device is a piece of equipment which typically provides a mechanical advantage in accomplishing a physical task, or provides an ability that is not naturally available to the user of a tool. The most basic tools are simple machines. For example, a crowbar simply functions as a lever. The further out from the pivot point, the more force is transmitted along the lever. When particularly intended for domestic use, a tool is often called a utensil.
Philosophers once thought that only humans used tools, and often defined humans as tool-using animals.
Some tools can also serve as weapons, such as a hammer or a knife. Similarly, people can use weapons, such as explosives, as tools.

Functions of tools
Often, by design or coincidence, a tool may share key functional attributes with one or more other tools. In this case, some tools can substitute for other tools, either as a make-shift solution or as a matter of practical efficiency. "One tool does it all" is a motto of some importance for workers who cannot practically carry every specialized tool to the location of every work task. Tool substitution may be divided broadly into two classes: substitution "by-design", or "multi-purpose" use, and substitution as make-shift. In many cases, the designed secondary functions of tools are not widely known. As an example of the former, many wood-cutting hand saws integrate a carpenter's square by incorporating a specially shaped handle which allows 90° and 45° angles to be marked by aligning the appropriate part of the handle with an edge and scribing along the back edge of the saw. The latter is illustrated by the saying "All tools can be used as hammers." Nearly all tools can be repurposed to function as a hammer, even though very few tools are intentionally designed for it.

Tool substitution

A Multitool is a hand tool that incorporates several tools into a single, portable device.
Lineman's pliers incorporate a gripper and cutter, and are often used secondarily as a hammer.
Hand saws often incorporate the functionality of the carpenter's square in the right-angle between the blade's dull edge and the saw's handle.