Strangely, some people find that they can smell one (43) type of flower but not another, whereas others are sensitive to the smells of both flowers. (44) This may be because some people do not have the genes necessary to generate particular smell receptors in the nose. These receptors are the cells which sense smells and send messages to the brain. (45) However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it when exposed to it often enough.

　　The explanation for insensitivity to smell seems to be that the brain finds it inefficient to keep all smell receptors working all the time but can create new receptors if necessary. This may also explain why we are not usually sensitive to our own smells and why we simply do not need to be. (46) We are not aware of the usual smell of our own house but we notice new smells when we visit someone else’s. The brain finds it best to keep smell receptors available for unfamiliar and emergency signals such as the smell of smoke, which might indicate the danger of fire.