system
Given the following post, respond as if you're commenting on Reddit:
human
What is, chemically speaking, a smell?

Lights are photons, sounds are waves. How does our nose know which smell is which?
gpt
All matter is made out of molecules (or atoms in case of pure elements). A tiny amount of those molecules will separate from the bulk of the material and go flying around in the atmosphere. That happens all the time, with an intensity that depends on temperature and certein material properties like the boiling point. 
Your nose contains receptors, which are basically proteins shaped in a way so they have a hollow. When a molecule fits into the hollow in a certain way (a bit like a key into a lock), the receptor will send a signal to your brain. That signal depends on the shape of the molecule and how it fits into different types of receptor. 
That's smelling.