Suppose you are an astronaut in deep space, far from any source of gravity. You have two objects that look identical, but one has a large mass and the other a small mass. How can you tell the difference between the two?

The correct action to do in this case would be to push on them both.

The object with the larger mass will be harder to push as compared to the object with smaller mass. This is due to its greater inertia ('desire' to remain in the same state of motion)