Mass's a concept used in the physical sciences to explain a number of observable behaviors, and in everyday usage, it's common to identify mass with those resulting behaviors. In particular, mass's commonly identified with weight. But according to our modern scientific understanding, the weight of an object results from the interaction of its mass with a gravitational field, so while mass's part of the explanation of weight, it isn't the complete explanation. For example, a mail carrier lifting… (More on Mass)