Here's why:

* Inertia is the tendency of an object to resist changes in its motion. It's a fundamental property of matter directly proportional to its mass.

* Mass is the amount of matter in an object.

Think of it like this: Imagine two identical cars, both weighing 1 ton. They have the same mass. It will take the same amount of force to get them moving from rest or to stop them once they're moving.

What can change how an object *appears* to have more or less inertia?

* Distribution of Mass: Even with the same mass, if the mass is distributed differently, the object might *feel* like it has more or less inertia. For example, a solid ball of clay is easier to spin than a ring of clay with the same mass. The ring has more inertia about its center axis.

* Shape and Form: A long, thin rod will be easier to rotate than a solid sphere with the same mass. This is because the mass is more concentrated in the sphere, making it harder to change its rotational motion.

* Friction: Friction can make an object seem to have more inertia. A heavy box on a rough surface is harder to move than the same box on a smooth surface. This isn't a change in the object's inertia itself, but rather an external force affecting its motion.

In summary: Mass and inertia are directly related. If two objects have the same mass, they have the same inertia. The distribution of mass, shape, and external forces like friction can influence how an object *appears* to have more or less inertia, but the underlying property of inertia remains the same.