How can we find out whether an egg is boiled or not, without breaking the shell?

    Newton's first law gives us the answer. The whole trick is that a boiled egg spins differently than a raw one. Take the egg, place it on a flat plate and twirl it. A cooked egg, especially a hard-boiled one, will revolve much faster and longer than a raw one; as a matter of fact, it is hard even to make the raw egg turn. A hard-boiled egg spins so ,quickly that it takes on the hazy form of a flat white ellipsoid. If flicked sharply enough, it may even rise up to stand on its narrow end.

     The explanation lies in the fact that while a hard-boiled egg revolves as one whole, a raw egg doesn't; the latter's liquid contents do not have the motion of rotation imparted at once and so act as a brake, retarding by force of inertia the spinning of the solid shell. Then boiled and raw eggs stop spinning differently. When you touch a twirling boiled egg with a finger, it stops at once. But a raw egg will resume spinning for a while after you take your finger away. Again the force of inertia is responsible. The liquid contents of the raw egg still continue moving after the solid shell is brought to a state of rest. Meanwhile the contents of the boiled egg stop spinning together with the outer shell.

Oleg Kobec