Two eyes, two ears… and one ginormous nostril? Human evolution might have worked that way (a loss to the nose ring industry, but a boon to Kleenex makers). But just in the last few years, researchers have discovered why two nostrils may be much better than one. Smelling in stereo, it turns out, actually helps us distinguish one odor from another. It also brings out a touch of the bloodhound in each of us.

To smell something, we need both nose and brain. About 2.75 inches (7 cm) up from the nostrils is a yellow, mucusy membrane called the olfactory epithelium. In this membrane are up to 40 million odor receptors, nerve cells studded with hairlike fibers called cilia. Each nerve cell (neuron) is connected by a longer fiber, called an axon, to the olfactory bulb, a part of the brain just above the nose.

When you smell something—fresh-popped popcorn at a movie theater, a box of new crayons, just-cut grass on a summer day—scent molecules have wafted through the air and up your nose. These tiny chemical fragments dissolve in the wet mucus of the epithelium, and make contact with the odor receptors.

Scientists think it’s the unique structure of each scent molecule that enables the brain to figure out what we’re smelling. Neurons in the epithelium send their excited electrical messages on to neurons in the olfactory bulb. The result is a map of each scent molecule, which the brain comes to recognize. Different smells are composed of different chemical "notes;" the brain decodes the entire scent recipe. Conclusion: popcorn!

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.

New research indicates that each nostril has an area of the brain dedicated to processing the molecules it picks up. Recent studies show that the two nostrils also help us figure out the direction scents are coming from. So, like a bloodhound dog—or a truffle-sniffing pig—we can figure out where the popcorn is, even if it’s just a few kernels.