Why is most of what we call taste actually smell?

Most of what you call taste is really smell, arriving by the back door. Your tongue only reports five things: sweet, salty, bitter, sour and umami. Telling an apple from a pear needs something else, and that something is retronasal smell. As you chew, odour molecules from the food rise from the back of your mouth into your nasal cavity, mostly as you breathe out, and reach the same smell receptors your nose uses for sniffing. Your brain then pins the result on your mouth, because chewing activates the motor cortex and the smell receptors together, so the smell feels like a taste.

You can catch the trick in the act. Pinch your nostrils while eating and the flavour fades, because the outgoing air path is blocked. A cold does the same thing: congestion shuts the passages, and your favourite dish suddenly tastes flat.

The psychologist Paul Rozin showed in 1982 that smell is really two senses. He trained people to recognize odours through the nose, then delivered the same odours to the back of the mouth, and their success fell drastically. His favourite example is Limburger cheese, repulsive to the nose yet pleasant in the mouth. Same molecules, different door, different verdict.

That may even be a human speciality. Dogs seem built for sniffing the world, while humans, with a short passage from mouth to nose, seem selected for this backward smelling.