Why Do Raw and Boiled Eggs Spin Differently?

Spin a raw egg and a hard-boiled egg on a smooth table. The boiled egg will usually rotate more steadily and for longer, while the raw one wobbles and slows sooner. The difference is not merely that one is “hard” and the other “soft”. It depends on whether the shell and its contents can rotate together as one body.

The inside of a boiled egg has set. When your hand turns the shell, the white, yolk and shell behave approximately as a rigid body and gain angular momentum together. In a raw egg, the shell moves first while the liquid initially lags because of inertia. Shearing within the liquid and friction between the liquid and shell convert some organised rotational energy into heat. The liquid and shell also keep exchanging angular momentum, making the rotation less stable.

A clearer test is to spin both eggs, touch each shell briefly to stop it, then release it at once. The boiled egg stays still because the whole object has stopped. The raw egg may begin turning again. Your finger mainly stopped the shell; for a brief moment the liquid inside kept moving, then transferred motion back to the shell through viscosity. The egg has not “remembered” the spin—the internal motion was never stopped at the same time.

This simple experiment shows that identical outer shapes do not guarantee identical dynamics. Before treating a system as one object, we need to ask whether its parts truly move together. Try it gently on a shallow plate or well away from the edge of a table so the eggs cannot roll off.

https://van.physics.illinois.edu/ask/listing/609
https://annex.exploratorium.edu/cooking/eggs/activity-spin.html