Here's why:

* Mass is the amount of matter in an object and is an intrinsic property. It stays the same regardless of location.

* Weight is the force of gravity acting on an object's mass. It's dependent on both the object's mass and the gravitational field it's in.

Example:

* An object with a mass of 10 kg will have a weight of 98 Newtons on Earth (where the gravitational acceleration is approximately 9.8 m/s²)

* The same object on the moon, where the gravitational acceleration is weaker, will have a lower weight.