Mass is a concept used in the physical sciences to explain a number of observable behaviors. In everyday usage, it is common to identify mass with the behaviors that result from it. In particular, mass is commonly identified with weight. But, in physics and engineering, weight means the strength of the gravitational pull on an object; that is, how heavy it is, measured in units of newtons. In normal situations, the weight of an object is proportional to its mass, which usually makes it unproblematic to use the same word for both concepts. However, the distinction between mass and weight becomes important for measurements with a precision better than a few percent (due to slight differences in the strength of the Earth's gravitational field at different places), and for places far from the surface of the Earth, such as in space or on other planets.