Which of the following is NOT a type of tooth?
Considering incisor, molar, canine, and dentin, the answer is: dentin.    
Photo credit: Encyclopedia Britannica. 

Dentin is the yellowish tissue that makes up the bulk of each tooth. It is harder than bone but softer than enamel, and it mainly consists of calcium and phosphate. Dentin sits in between the pulp of a tooth and outer enamel.

Though they may be your pearly whites, teeth aren't actually pure white bone all the way through. In fact, they are comprised of a number of different anatomical features that allow for full functionality when chewing, biting and smiling.

The two main parts of the tooth are the crown and the root— or simply, the top and the bottom. The crown refers to the part of the tooth we can see, and is primarily comprised of enamel and dentin. Enamel is actually the strongest substance in our body— even stronger than bone! This is due to the fact that it is 96 percent mineral, the highest percentage of any tissue in your body, which in turn makes it durable and damage-resistant. Enamel covers the crown of your tooth like a helmet, protecting it from the wear and tear of every day use.

The root, or bottom of your tooth, is embedded in a socket called your gum. The root serves as the tooth's anchor, and houses the pulp, or nerve endings, in the tooth. Like the crown, though, the root also has a protection system in place: cementum. Cementum covers the root of the tooth and works like a softer type of enamel. This substance helps keep the root stable by fusing to the fibers that connect the tooth to the jawbone. By keeping the pulp safe, the tooth continues to receive blood and nutrients to keep it alive.

Learn more about the anatomy of a tooth here.

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