Tooth enamel is the hardest substance produced by the human body.
Why it is true
Composed mostly of minerals like hydroxyapatite, enamel is harder than bone and serves to protect teeth from the extreme forces of chewing.
Source: Dental Materials
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.