Ask a grown-up: why do we have two nostrils?
Well, partly it's a developmental thing: we have a pair of every feature that is not mid-line in the body – so, two eyes, two ears, two nostrils (yes, we have only one mouth, but that, too, follows the same structure). This happens because early on, when the human body develops, it forms almost from a flat plate that folds itself in two.
Another reason we have two nostrils is because of something called the "nasal cycle". While we don't consciously notice it, we breathe predominantly through one nostril at a time, and the other side becomes a bit congested and rests. You can test this yourself: try breathing through one nostril and then the other, and you'll probably find that one becomes stuffier than the other. This automatically changes over about four to six hours, so then the other side relaxes.
Finally, imagine how odd we'd look with just the one nostril.
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