You can tell whether an egg is hard-boiled just by spinning it!
If the egg spins smoothly, fast, and it easy to stop, it is hard-boiled. However, if the egg is wobbly and somewhat difficult to spin or stop, it is still raw. This is because the liquid inside a raw egg does not spin quite as fast as its shell. This delays both its starting and stopping and makes its spinning motion a bit awkward and irregular. You don’t have this problem with hard boiled eggs because all the material inside is uniformly distributed and spins just as fast as the outer shell!
