Here's why:

* Mass is a fundamental property of an object, representing the amount of matter it contains. It's a scalar quantity and doesn't change regardless of location.

* Weight, on the other hand, is a force. It's the force exerted on an object due to gravity. It's a vector quantity, meaning it has both magnitude and direction.

The relationship between mass and weight is:

Weight = Mass × Acceleration due to gravity

So, while mass remains constant, weight can change depending on the gravitational field strength. For example:

* An object on Earth has a certain weight due to Earth's gravity.

* The same object on the Moon would have a lower weight because the Moon's gravity is weaker.

Therefore, weight is the direct measure of the force of gravity acting on an object.