Why Can Spinning an Egg Reveal Whether It Is Raw or Cooked?

Spin a raw egg and a hard-cooked egg on a smooth bench. The cooked egg will usually rotate more steadily, while the raw one tends to wobble and slow sooner. The main difference is not the shell but whether the material inside can turn with it.

In a hard-cooked egg, the white and yolk have set, so the shell and contents respond approximately as one solid body when your hand gives them a twist. A raw egg contains liquid. The shell moves first; viscosity gradually drags the liquid near the shell, while fluid closer to the centre lags behind. Relative motion between the layers dissipates some organised rotational energy as heat, so less of the hand’s twist becomes steady motion of the whole egg.

There is an even clearer test. Set both eggs spinning, touch each briefly to stop its shell, then release it immediately. The cooked egg stays still. The raw egg may begin turning again because its liquid interior did not come fully to rest during the brief touch. Once released, that moving fluid drags the shell around again. The egg has not “remembered” the spin; angular momentum remained inside it.

The experiment shows how objects with the same exterior can behave differently because of their internal state. It is a demonstration of rotation and internal dissipation, not a substitute for cooking-time and temperature guidance for food safety.

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