When you sniff an odor you probably believe that your experience is a direct result of what you are smelling—that is, the stimulus dictates what you smell. Many olfactory scientists held this commonsense view until fairly recently, but it is looking increasingly wrong. Rather, what one smells appears to be defined by what the stimulus reminds your olfactory system of. If you smell caramel, for example, it smells sweet, yet sweetness is a quality associated with a completely different sensory system—taste. Therefore this experience of a “tasty smell” must be a consequence of memory. This point is made most strikingly when olfactory memory itself is damaged, leaving patients able to smell but unable to experience an odor's unique quality. That is, roses and gasoline, ...