How can you tell a boiled egg from an uncooked egg? This experiment works well on an OHP as a classroom demonstration. Spin a hard-boiled egg and a raw egg on their ends, as fast as you can. Stop them by placing your finger on the shell and removing it quickly.
The two eggs will behave rather differently. The boiled egg will spin steadily, and is easily stopped by touching it. The raw egg will start spinning again once it is released. This is because the liquid inside the egg is still moving, due to inertia.
Inertia is the property of a body that means it will continue moving in a straight line (or remain at rest) if there is no unbalanced force acting on it. This is the basis of Newton's famous first law of motion.
There are lots of other experiments you can use to demonstrate inertia. The brave at heart can try pulling a tablecloth off a table laden with crockery, without disturbing the crockery. If you pull the cloth quickly enough, the experiment works very well.
A safer alternative is to make a stack of pennies. Using a metal spatula you should be able to remove the bottom penny without knocking the stack over. The motion needs to be quick and accurate, and may require some practice. However, you can then remove the coins one at a time in quick succession.
Please note that it is important to ensure that your students are a safe distance away, or they may be injured by a flying coin.
Alternatively, place a postcard on top of a plastic cup, and balance a coin in the middle. Flick the card quickly, and the coin will fall into the cup.
Or place a piece of paper under a tower of blocks and remove it by pulling it quickly away. If you pull quickly enough the tower will remain standing.
All these experiments require practice - but they give great results.