You 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 ( 1 ) under any condition? In spite of our ( 2 ) discomfort, it turns out there9s a good reason our teeth are so sensitive. Tooth pain is a ( 3 )mechanism that ensures when a tooth is being damaged we'll notice and do something about it.<p>If we eat something too hot or too cold, or if the tooth is worn down enough where the tissue ( 4 ) 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 might ( 5 )to use them in situations that damage them, and for humans, damaging ( 6 ) teeth is a problem because, unlike crocodiles, we can't ( 7 )them.</p><p>Teeth have three layers, only one of which-the innermost layer of the tooth-can hurt, as that layer of the tooth ( 8 )both 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 ( 9 )by heat or cold, the nerves in the inner layer don't sense temperature. Rather, they feel pain, which may be( 10 )with, say, drinking something very cold.</p><p>A) adult F) defense K) mammal</p><p>B) associated G) dental L)replace</p><p>C)chew H) downward M) swallow</p><p>D) contains I) emotional N) triggered</p><p>E) continue J) implies 0) underneath</p>