When the hardboiled egg is spun, the solid centre has a fixed centre of gravity, causing it to move at the same time as the shell. This results in a smooth, balanced spin.

How do you tell if an egg is hard boiled by spinning?

If you're wondering how to tell an egg is hard boiled, set it on the counter and give a quick spin. Once it's moving, tap your finger on it to stop the spinning. Eggs that are cooked will spin easily and rapidly and stop quickly.

Does a good egg spin?

Take the egg and spin it on a flat surface; if the egg wobbles, it's fresh (the insides are moving around). If the egg spins smoothly, it's cooked. Whether your eggs are okay to use or not, you will still be left with the shells and carton.