The human nose is an underrated tool.Humans are often thought to be insensitive smellers compared with animals,(1) _____ this is largely because, unlike animals, we stand upright.This means that our noses are (2) _____ to perceiving those smells which float through the air, missing the majority of smells which stick to surfaces.In fact, (3) _____, we are extremely sensitive to smells, even if we do not generally realize it.Our noses are capable of (4) _____ human smells when these are diluted to far below one part in one million.

　　Strangely, some people find that they can smell one type of flower but not another, (5) _____ others are sensitive to the smells of both flowers.This may be because some people do not have the genes necessary to generate (6) _____ smell receptors in the nose.These receptors are the cells which sense smells and send (7) _____ to the brain.However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it when (8) _____ to it often enough.

　　The explanation for insensitivity to smell seems to be that the brain finds it (9) _____ to keep all smell receptors working all the time but can (10) _____ new receptors if necessary.This may also explain why we are not usually sensitive to our own smells-we do not need to be.