Tooth enamel is the hardest substance in the human body.
Why it is true
Tooth enamel is the hardest substance in the human body, with a hardness of 5 on the Mohs scale. It is made of calcium phosphate and covers the crown of the tooth, protecting it during chewing. However, it does not regenerate.
Source: American Dental Association
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.