Here's why:

* Inertia is the tendency of an object to resist changes in its motion. A stationary object wants to stay stationary, and a moving object wants to keep moving at the same speed and direction.

* Mass is a measure of how much matter an object contains.

The more mass an object has, the harder it is to get it moving or to stop it once it's moving. This means that greater mass implies greater inertia.

Think of it this way:

* A small, lightweight object (like a feather) is easy to push and stop. It has low inertia.

* A large, heavy object (like a car) is much harder to push and stop. It has high inertia.

The relationship between mass and inertia is a fundamental concept in physics, and it's expressed in Newton's First Law of Motion: an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.