[00:03.12]2005 [00:06.89]The human nose is an underrated tool. [00:09.60]Humans are often thought to be insensitive smellers [00:12.53]compared with animals, [00:14.04](1)but this is largely because, (2)unlike animals, [00:16.97]we stand upright. [00:18.69]This means that our noses are (3)limited to [00:20.81]perceiving those smells which float through the air, [00:24.03](4)missing the majority of smells [00:25.54]which stick to surfaces. [00:27.66]In fact, (5)though, we are extremely sensitive to smells, [00:31.59](6)even if we do not generally realize it. [00:34.42]Our noses are capable of (7)detecting human smells [00:37.44]even when these are (8)diluted to far below [00:40.06]one part in one million. [00:42.68]Strangely, some people find that they can smell [00:45.71]one type of flower but not another, [00:47.72](9)whereas others are sensitive [00:49.14]to the smells of both flowers. [00:51.35]This may be because some people do not have [00:53.46]the genes necessary to generate (10)particular smell receptors [00:57.12]in the nose. [00:58.00]These receptors are the cells which sense smells [01:01.19]and send (11)messages to the brain. [01:03.61]However, it has been found [01:05.22]that even people insensitive to [01:06.84]a certain smell (12)at first [01:08.66]can suddenly become sensitive to it [01:10.47]when (13)exposed to it often enough. [01:13.30]The explanation for insensitivity to smell [01:16.01]seems to be that the brain finds it [01:17.93](14)inefficient to keep all smell receptors working all the time [01:21.77]but can (15)create new receptors if necessary. [01:24.50]This may (16)also explain [01:25.91]why we are not usually sensitive to our own smells [01:29.14]--we simply do not need to be. [01:30.96]We are not (17)aware of the usual smell [01:33.11]of our own house, [01:34.52]but we (18)notice new smells [01:35.93]when we visit someone else's. [01:37.84]The brain finds it best to [01:39.37]keep smell receptors (19)available for unfamiliar [01:42.09]and emergency signals [01:43.71](20)such as the smell of smoke, [01:45.11]which might indicate the danger of fire.

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