Smelling means detecting molecules
Many substances release tiny molecules that enter the air. When you breathe in, some reach a small region of specialised tissue high inside the nasal cavity: the olfactory epithelium. Odour molecules dissolve in the mucus there and interact with receptor proteins on olfactory sensory neurons.
From a molecule to a nerve signal
Humans have hundreds of types of olfactory receptor. Each sensory neuron typically expresses one receptor type, and an odorant can activate more than one receptor. The result is a pattern of activity across many neurons, rather than a one-molecule/one-smell code.
When receptors are activated, a biochemical signalling cascade changes the neuron’s electrical activity. Signals travel to the olfactory bulb, where information is organised and relayed to other brain regions. The brain combines this activity with context, memory and other sensory information to produce a perception.
Why a smell can bring back a memory
Smell is closely connected to brain systems involved in emotion and memory. A familiar odour can therefore evoke a vivid memory or feeling. That association is not a property stored in the molecule itself; it arises from how the nervous system processes and links experiences.
Why mixtures are difficult to describe
A cup of coffee contains many volatile compounds. The receptors and neural circuits respond to combinations, and one component can alter how another is perceived. This is why the smell of a mixture is not always the simple sum of its ingredients.
The surprising part: your nose does not contain a separate labelled receptor for every possible smell. It detects combinations, and the brain interprets the resulting pattern.
Further reading
U.S. National Institute on Deafness and Other Communication Disorders: Smell disorders (includes a section on how the sense of smell works).