Here's why:

* Inertia is the tendency of an object to resist changes in its motion. This means an object at rest wants to stay at rest, and an object in motion wants to stay in motion at the same speed and direction.

* Mass is a measure of the amount of matter an object contains.

* The greater the mass of an object, the greater its inertia. This means it takes more force to change the object's velocity (start it moving, stop it, or change its direction).

Think of it this way: a bowling ball has more mass than a tennis ball. You'll need more force to get the bowling ball moving, and you'll have a harder time stopping it once it's rolling. This is because the bowling ball has more inertia due to its greater mass.

In summary: mass is the property that determines how much inertia an object possesses. The more mass an object has, the more resistant it is to changes in its motion.