Apparently in the 1940s there was this guy named Amoore, and he came up with what he considered to be the seven basic odors. They were camphoraceous (like moth balls), musky (musk colonge), floral (flowers), minty (mint), etheral (ether), pungent (vinegar), and putrid (sulfur). He looked at different chemicals and found that there seemed to be a relationship between the shape of a molecule and the scent sensation that it produced. Camphoraceous molecules were shaped like an egg, musky molecules like an oval disk, floral ones like a mace (the weapon used in medival times that consisted of a sphere with pointed spikes protruding from it), minty molecules like an oval wedge, and etheral ones like a rod. The pungent and putrid molecules didn't seem to have a certain shape to them, but pungent molecules were positively charged while putrid ones were negatively charged. After figuring this out, Amoore synthesized some new molecules with each of these characteristics and had some "expert sniffers" try them. These synthetic molecules generally produced scent sensations similar to natural molecules of the same shape. Thus, it was proposed that olfaction works by a kind of lock-and-key method, so the molecule bumps into a certain receptor, and if the molecule fits, it causes some sort of conformational change in the receptor and a signal is conducted and we smell, say, a flower. Think of that next time someone tells you to "take time to stop and smell the roses."