The olfactory epithelium contains special receptors that are sensitive to odor molecules that travel through the air. In humans, the olfactory epithelium is located about 7 cm up and into the nose from the nostrils. Chemical molecules enter the nose and dissolve in mucous within a membrane called the olfactory epithelium. The sense of smell, called olfaction, involves the detection and perception of chemicals floating in the air. Those molecules are generally light, volatile (easy to evaporate) chemicals that float through the air into your nose. Everything you smell, therefore, gives off molecules.

Many odours are not single scents or single kinds of molecules but a whole mixture of them. What makes up the smell of something? Smells or odours are composed of tiny molecules of chemicals invisible to the human eye from things like food, flowers or poo. Sometimes the sense of smell can be fooled because humans do not get as full and complete a picture from smell as we do with vision. In this activity, students try to identify substances using only their sense of smell.