"In humans, the perception of odours adds a fourth dimension to life, from the scent of flowers, the aroma of foods, and all the subtle smells in the environment. But how many types of odours can we distinguish? Why do we like the food we like? Which are the most powerful odorants, and how well does the human sense of smell perform compared with that of a dog or a butterfly? The sense of smell is highly complex, and such complexity discouraged scientists...