What is the primary characteristic of a lever?

The primary characteristic of a lever is that it can rotate around a fixed point, known as the fulcrum. This rotational movement allows a lever to amplify an applied force, making it easier to lift or move heavy objects. Essentially, a lever transforms the input force applied at one end into a larger output force at the other end, depending on the distances from the fulcrum. This principle is fundamental to how levers are used in various applications, from simple tools to complex machines.

The other options do not accurately describe the core characteristic of a lever. The principle of buoyancy pertains to fluids and is unrelated to the mechanical advantage offered by levers. The notion that a lever is affected only by external forces does not capture its function, as the internal mechanics of the fulcrum and the lever arm are also critical to its operation. Lastly, the generation of electricity through motion describes concepts found in generators or alternators, which operate on different principles than those associated with levers.