Inertia: Knock the Penny Out

Make a stack of 5 or 6 pennies. Use another penny to try to knock the bottom penny out, while keeping the rest of the stack in tact.

Go a step farther and make your stack of pennies on a piece of paper. Trace a circle around the pennies. Now, try to keep the stack of pennies in the circle after you knock the bottome one out.