First, let's talk about how smell works. From coffee to stinky trash, the substances around us give off volatiles, which you can think of as tiny smell molecules. We breathe in these smell molecules, which then stimulate the olfactory sensory neurons that sit high in the nose. Each of these neurons contains an odor receptor on its surface. Once the odor receptors are triggered by these smell molecules, the neurons send information about them to the brain. Here's what I think is so cool. The brain not only categorizes that information as a particular odor, it may also begin to associate feelings like pleasure or disgust and other moods and emotions with that odor for future reference. For example, you sniff bacon. You eat it. Your taste buds get salt, and then your body gets a whack of fat, which is an energy source. So the brain loves it and attaches a positive label to it. The next time you smell bacon, a sensation associated with pleasure arises.