The sense of smell is a very distant fifth place in our senses: sight, hearing, taste, and touch all come before smell in our thoughts. Because of that, we underestimate both its sensitivity and its influence. Our sense of smell is what makes food tasty and repels us from rotting things. Our sense of smell evokes some of our strongest memories.

But frankly, the sense of smell is confusing.

At one level, sensing odors is easy: a molecule sticks to something in our nose. The thing it sticks to then sends a signal to the brain, which instructs our hand to grab the last cookie. But those two sentences sweep all the complexity under the carpet. Why does a molecule stick? And how do the nose (and brain) distinguish different molecules?