Inform your audience that the concept of inertia will help you identify a raw egg from a hard-boiled one.
Ask an audience member or classmate to select two eggs. One should be hard-boiled and one should be uncooked.
Spin each egg quickly on a flat, smooth surface. Do not spin them at the same time, as the raw egg may crack if the hard-boiled egg hits it.
Stop each egg mid-spin by touching it gently with your fingers. Apply just enough pressure to stop the egg.
Release the egg immediately once it stops spinning. The hard-boiled egg will stay stopped but the raw egg will continue spinning or rocking.
Explain that the hard-boiled egg is a solid mass inside the shell, so when you stopped the shell, you stopped everything inside it. However, the raw egg has liquid inside it and the liquid did not stop moving just because the shell did. In other words, you did not apply enough outside force to stop a body that was already in motion.
Prove this concept beyond a doubt by cracking open the egg you identified as the hard-boiled one and eating it.