Suppose you had to distinguish between a raw egg and a hardboiled one - without cracking them open to see which was which. You might be stumped, unless you knew this old parlour trick: lie both eggs down on their sides on a table, and spin them as you would a spinning top. Try it a few times, and with luck, you'll get the cooked egg to rise up for a few seconds on one end as it spins. The uncooked one won't do any such thing.