But if the same scent molecules drift up both nostrils when we take a sniff of, say, a rose, why isn’t one enough? It turns out that tissue lining each nostril shrinks or expands with blood flow. When one side is slightly swollen, the other is free-breathing. This cycle goes on day and night, making the airflow through each nostril slightly different. (Test this for yourself by pressing a nostril shut and sniffing through the other side. Now switch.)
In the last few years, researchers think they have solved part of the "two nostrils" puzzle. Some scent molecules dissolve quickly in mucus; others take longer. So slow-dissolving molecules that are whisked too quickly through the nose don’t have a chance to register. Meanwhile, fast-dissolving molecules have their biggest scent impact when they are swept into contact with a large swath of neurons. When we inhale a scent, having two nostrils with different airflows allows us to better detect both kinds of odor molecules, giving us a more complete smell picture of an object.