Here's why:

* Mass is the amount of matter in an object. It's a fundamental property of an object and doesn't change based on location or gravity.

* Weight, on the other hand, is the force of gravity acting on an object's mass. It can change depending on the gravitational field.

A balance works by comparing the force of gravity acting on two objects (the object you want to measure and a known weight). By balancing the forces, you can determine the mass of the object.