If you were to take out an old filling, usually there's just a shell left. So that's a very weak substructure, so a crown is actually something that covers and protects the tooth.

They may give you a whiter grin at first. But over time, they can wear down your teeth's hard outer shell-- enamel. That can make your teeth more sensitive to hot or cold food and drinks.

PRESENTER:The shell of an egg is a bit like the coating on our teeth we call the enamel. The shell protects the egg just like the enamel protects our teeth. PRESENTER:I'm going to put this egg ...