Here's why:

* Mass is the amount of matter in an object.

* Triple-beam balances work by balancing the object's weight against known weights. The weight is a force due to gravity acting on the mass. By balancing the forces, you can determine the mass of the object.

It's important to note that a triple-beam balance doesn't directly measure weight. It measures mass, which is a fundamental property of matter. Weight is a force that depends on both mass and the acceleration due to gravity.