Dentin (also spelled dentine) is the hard, yellowish material that makes up most of a tooth, lying just beneath the white enamel coating you can see when you smile. While enamel is the hardest substance in your body, dentin is slightly softer but still incredibly strong and durable.

Think of a tooth like a house: the enamel is the tough outer walls, but dentin forms the main structure inside. Dentin is actually alive, containing tiny tubes that connect to the nerves in the center of your tooth. This is why a cavity that breaks through the enamel and reaches the dentin can start to hurt: those tubes let you feel temperature and pressure.

Dentin is what gives teeth their natural color. Even healthy teeth look slightly yellow because you're seeing the dentin through the translucent enamel. Animals have dentin too: elephant tusks are made almost entirely of dentin, which is one reason ivory was once so highly valued. Understanding dentin helps explain why dentists work so hard to protect your enamel, since once bacteria reach the dentin layer, tooth decay can spread more quickly through those microscopic tubes.