The human nose can distinguish over one trillion scents.
Why it is true
Recent studies have shown that the human olfactory system is much more sensitive than previously thought, capable of distinguishing at least one trillion different odour stimuli.
Source: Rockefeller University
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.