If you hold up two eggs and one is hard-boiled and the other is raw, you might wonder how to know which is which.
A simple test will reveal the answer. Spin them carefully on a countertop. The hard-boiled one spins and the raw one doesn't. This is because the hard-boiled egg is solid so everything spins in one direction, while the inside of the raw egg sloshes in different directions and, therefore, doesn't allow it to spin. Try it and see for yourself.