One thing scientists study is how objects move. A famous scientist named Sir Isaac Newton (1642-1727) spent a lot of time observing objects in motion and came up with three laws that describe how things move. This explanation only deals with the first of his three laws of motion. Newton's First Law of Motion says that moving objects will continue moving in the same direction at the same speed. It also says that objects at rest will remain at rest unless an outside force acts on them. The idea of an object resisting change in its motion is called inertia. All objects have inertia. The larger the mass of an object, the more inertia it has. It is much harder to get a large boulder moving than a tiny pebble. The boulder has a greater amount of mass and therefore more inertia than the pebble. The activity described in this article will help students understand the scientific principle of inertia.