Our sense of smell is probably the oldest of the five senses. As you breathe in, air passes through a cavity behind the nose which contains patches of millions of smell receptors called factory cells. Sensory hairs stick out form the surface of these receptor cells and these hairs detect smells and pass information along nerve fibers to the brain.
Substances that you can recognize as having an odour dissolve in the layer of mucus covering the sensory cells, stimulating them to produce a signal. Most people are able to detect about 4,00 different smells. However, people whose work is based on their ability to smell, such as chefs, perfume makers and wine tasters, can distinguish as many as 10,00 different smells.