What's happening?

Inertia is a property of all things that makes them resist any change in their motion. When an object is standing still, it takes a force to get it moving. And once it is moving, it takes a force to stop it.

The heavier an object is, the more force-or time-it takes to change its momentum. That 's the key to doing this trick. You can change the momentum of one part (the light paper) without changing the momentum of the other (the heavy pennies).

• Return to the experiment.

Excerpt from The Jumbo Book of Science by the Ontario Science Centre, used by permission of Kids Can Press Ltd., Toronto.