Telling a boiled egg from a raw one without breaking it
What liquid inside does to rotation, and what that reveals from outside.
Spinning a boiled egg and a raw egg side by side and giving each the same flick tells them apart immediately, because a boiled egg is solid all the way through and spins as one rigid piece, while a raw egg's liquid interior lags behind the shell's spin and drags against it, producing a slower, wobblier spin even though both eggs look identical from outside.
The mechanism behind the spinning-egg test
A boiled egg is solid protein all the way through, its white and yolk both set firm by heat, so when the shell is spun every part of the egg is forced to spin along with it at exactly the same speed, the way any single rigid object does. A raw egg's shell contains liquid white and yolk that are not attached to the shell at all, only resting inside it, and when the shell is flicked into a spin the liquid does not instantly start spinning with it, it takes a moment to catch up, dragged along gradually by friction against the inside of the shell rather than moving as one piece with it from the first instant. That lag matters twice over: while the egg is spinning, the still-catching-up liquid acts as a drag on the shell, slowing and wobbling the spin compared to the boiled egg's clean, steady rotation, and if the spinning shell is touched to stop it and then released, the liquid inside, having finally caught up to speed, is still moving and drags the stopped shell back into a brief spin of its own. Neither effect has anything to do with which egg is heavier or which spins faster from the same flick, both start out roughly the same, the difference only appears once the spin is under way and the liquid inside the raw egg begins, or fails, to keep pace with its own shell.
The sloshing-bucket comparison
Carrying a half-filled bucket of water and turning around quickly shows the same lag on a larger scale. The bucket itself changes direction the instant your arm does, but the water inside does not, it keeps moving in roughly its old direction for a moment, sloshing up one side of the bucket before friction and the bucket's walls drag it around to match. A raw egg spun by a flick of the fingers is a bucket of water on a much smaller and faster scale, and the same lag between the container's motion and the liquid's is exactly what makes the spin feel loose and wobbly rather than clean and steady the way the boiled egg's spin does. A bucket carried gently, turned only a little at a time, barely sloshes at all, in the same way a raw egg flicked only lightly shows less of the wobble than one spun hard, since a smaller, gentler push gives the liquid less of a gap to fall behind by in the first place.
The one number worth remembering
The clearest version of the test needs no careful observation of speed at all, only a light touch to stop the shell for a moment and a quick release: a boiled egg, entirely solid, stays stopped, while a raw egg, its still-moving liquid dragging the shell back into motion within a fraction of a second, visibly starts spinning again on its own the instant the fingers lift away. No scale, timer, or careful measurement is needed to see it, only a flat surface, a flick of the fingers, and a moment's patience to watch what each egg does once the hand pulls back.
What this changes in practice
Because the difference comes from whether the inside of the egg is rigidly attached to the shell or merely sitting loose inside it, the same test works on anything built the same way, and a sealed container can be checked for a solid fill versus a liquid one by spinning and briefly stopping it, without ever needing to open it to find out which is inside. It also explains why a raw egg spun hard enough for long enough eventually starts spinning almost as cleanly as a boiled one, since given enough time the liquid inside does finally catch up to the shell's speed and stops lagging behind it, only for the whole thing to slow down again as friction with the table takes over before it can settle for long.