Here's why:

* Inertia: Inertia is the tendency of an object to resist changes in its motion. This means it wants to stay at rest if it's at rest, and it wants to keep moving at a constant velocity if it's already in motion.

* Mass: Mass is the amount of matter in an object. The more massive an object is, the harder it is to accelerate or decelerate.

In simpler terms:

Think about pushing a small toy car and a large truck. The toy car is easy to get moving and to stop. The truck is much harder to get moving and to stop because it has more mass. The truck has more inertia.

Key Point: Mass is the direct measure of inertia. The larger the mass, the larger the inertia.