Smell is such a powerful and revealing sense because it detects actual little pieces of things in the world. It gives us direct evidence of what those things are made of-unlike the indirectness of vision or hearing, which register light waves and air movements. Those little pieces are volatile molecules, so little that they're able to break away from their source and fly invisibly through the air to reach our nose. To begin to understand a thing's smell, then, is to identify the many volatile molecules it emits. Its overall smell is a composite, created by the component smells or "notes" of its most prominent volatile molecules. When different things seem to echo each other with shared component smells, it's a sign that those things have some volatile molecules in common. And the chemical identities of the molecules are keys to why they're there. They're tokens of the processes that created them. Text and 200 tables cover this topic, in a book by an expert on the chemistry and history of food science and cooking.