The human body contains about 37.2 trillion cells.
Why it is true
This estimate was calculated in 2013 by Israeli researchers. Most are red blood cells.
Source: Annals of Human Biology 2013
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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.