十五选十A)adult B) associated C)chew D) contains E) continue F) defenseG)dental H) downward I) emotionalJ) implies K)mammal L)replaceM)swallow N) triggered O) underneathYou might think of your teeth as tools, like built-in knives and forks, but if they are mere tools, why do they feel pain and wouldn't it be better if they could just 26 under any condition? In spite of our 27discomfort, it turns out there's a good reason our teeth are so sensitive. Tooth pain is a 28 mechanism that ensures when a tooth is being damaged we'll notice and do something about it.If we eat something too hot or too cold, or if the tooth is worn down enough where the tissue 29 is exposed, all of those things cause pain, and then the pain causes the person not to use that tooth to try to protect it a little bit more. So it's really a protective mechanism more than anything else. If teeth didn't feel pain, we might30to use them in situations that damage them, and for humans, damaging 31 teeth is a problem because, unlike crocodiles, we can't 32 them.Teeth have three layers, only one of which-the innermost layer of the tooth-can hurt, as that layer of the tooth 33both blood vessels and nerves. Pain is the only feeling to which the nerves in that layer respond. Whereas people with tooth sensitivity may complain, for example, of tooth pain 34 by heat or cold, the nerves in the inner layer don't sense temperature. Rather, they feel pain, which may be 35 with, say ,drinking something very cold.