Here's why:

* Mass is a fundamental property of matter that represents the amount of "stuff" in an object. It's a measure of inertia (resistance to changes in motion) and is independent of location.

* Weight, on the other hand, is the force exerted on an object due to gravity. Weight does change on different planets because the gravitational force varies depending on the planet's mass and radius.

Example:

Imagine you have a 100 kg person on Earth. They would still have a mass of 100 kg on Mars, the Moon, or any other planet. However, their weight would be different because the gravitational force is different on those planets.

Let me know if you'd like more details about weight and how it differs on different planets!