Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: The inertia of an object depends on its mass. Objects with greater mass also have greater inertia. Think how hard it would be to push a big box full of books, like the one in Figure below . Then think how easy it would be to push the box if it was empty. The full box is harder to move because it has greater mass and therefore greater inertia. Answer: inertia
Output:
What property increases as an object's mass increases, making a full box harder to push than an empty one?