A modern hammer is directly descended from ancient hand tools 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.[citation needed] But observation has confirmed that multiple species can use tools, including monkeys, apes, several corvids, sea otters, and others. Later, philosophers thought that only humans had the ability to make tools, until zoologists observed birds[1] and monkeys[2][3][4] making tools. Now humans' unique relationship to tools is considered to be that we are the only species that uses tools to make other tools.[citation needed] Most anthropologists believe that the use of tools was an important step in the evolution of mankind.[5] Humans evolved an opposable thumb - useful in holding tools - and increased dramatically in intelligence, which aided in the use of tools.[6] Some tools can also serve as weapons, such as a hammer or a knife. Similarly, people can use weapons, such as explosives, as tools.