Our sense of smell is remarkably potent. Freshly mown grass, pine needles, Camembert cheese, burning rubber - even the memory of such smells can evoke powerful responses.
High inside our noses sit two patches of cells that act as smell receptors. The two patches comprise millions of cells, each with minute hair-like projections waving in a sea of mucus, like a mat of wafting reeds in a riverbed. These hairy cells, called cilia, are incredibly sensitive. A single molecule of some substances is enough to excite them into sending a message to the brain.
There are at least 14 different kinds of smell receptor cell, each of which is excited by a different type of smell molecule. This allows our brains to work out not only that something smelly has gone up our noses, but exactly what it is. Most familiar smells - freshly made coffee, cigarette smoke, and delicate perfumes - are complex mixtures of odours.
Sum better than its parts
Some perfumes that we find highly desirable are made up of substances that on their own smell quite offensive. Civet, for example which comes from the anal glands of a wild cat, has a vile smell in itself, yet it is a vital element in most expensive perfumes.
Humans are able to distinguish between more than 10 000 complex odours. Surprisingly, we do not seem to put this ability to any very significant use. Some scientists now believe that smells play an important hidden role in the relations between people, creating unconscious bonds. Experiments have shown that babies can already distinguish between their mothers and strangers at the age of six days using their sense of smell.