The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【B1】 this is largely because, 【B2】 animals, we stand upright. This means that our noses are 【B3】 to perceiving those smells which float through the air, 【B4】 the majority of smells which stick to surfaces. In fact, 【B5】 , we are extremely sensitive to smells, 【B6】 we do not generally realize it. Our noses are capable of 【B7】 human smells even when these are 【B8】 to far below one part in one million. Strangely, some people find that they can smell one type of flower but not another, 【B9】 others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate 【B10】 smell receptors in the nose. These receptors are the cells which sense smells and send 【B11】 to the brain. However, it has been found that even people insensitive to a certain smell 【B12】 can suddenly become sensitive to it when 【B13】 to it often enough. The explanation for insensitivity to smell seems to be that brain finds it 【B14】 to keep all smell receptors working all the time but can 【B15】 new receptors if necessary. This may 【B16】 explain why we are not usually sensitive to our own smells—we simply do not need to be. We are not 【B17】 of the usual smell of our own house, but we 【B18】 new smells when we visit someone else's. The brain finds it best to keep smell receptors 【B19】 for unfamiliar and emergency signals 【B20】 the smell of smoke, which might indicate the danger of fire. 【B1】