Spinning reveals the difference. Take a raw egg and a hard-cooked egg, spin each egg rapidly for a while, then stop it abruptly. When you let go, the raw egg will slowly start to move by itself. The hard-cooked egg will stay put. Because the components in a raw egg are different densities, the egg tends to wobble reluctantly rather than spin quickly like the hard-cooked egg. And when the egg is stopped, the fluid inside continues to rotate. The hard-cooked egg with its solid components is able to come to an abrupt stop.
