Humans have a very keen sense of smell: we can detect thousands of different smells. This ability relies on the presence of special sensory receptors in the upper part of the nose. When stimulated by odour molecules, these receptors send signals along nerves to the brain for processing. Sometimes odour molecules do not reach the sensory area, but sniffing will help get them there.
Smell receptors are specialized nerve cells. Each bears many tiny cilia (hairs), which project into the space in the upper part of the nose. A nerve fibre extends from the other end of each cell. This joins other fibres to form the olfactory nerves, which carry signals to the brain.
The cilia can detect tiny amounts of substances in the air, though molecules of those substances must first be absorbed by the mucus layer. There they interact with the cilia to trigger nerve impulses.