Here's why:

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

* Weight is the force of gravity acting on an object's mass. It's dependent on the gravitational field strength of the location.

So, while mass remains constant, weight can change depending on the gravitational field. For example, you'd weigh less on the moon than on Earth because the moon has a weaker gravitational pull.