2020托福听力练习：海狸牙齿好的奥秘（下载）

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　　Ah yes.The ol' fluoride rinse at the dentist.Not pleasant.

　　But hey, good for your teeth, right?

　　Well now materials scientists have been able to figure out why—by mapping the nanostructure of tooth enamel.

　　If you zoom way in, tooth enamel looks almost like the weave of a basket.

　　"Where each thread is made from thousands of nanowires."

　　Derk Joester, of Northwestern University.

　　And in between those crystalline nanowires,Joester and his colleagues discovered a sort of amorphous glue.

　　And that's where the fluoride hangs out, helping to stave off an acid attack of the enamel—in other words, a cavity.

　　But the researchers found something that works even better than fluoride: iron.

　　And they found it in beaver teeth.

　　"Beavers don't get caries.

　　Chewing through wood is a very good way to clean your teeth."

　　But another reason, they say, is the iron-enriched glue in beaver enamel—which was even more acid-resistant than fluoride-treated enamel.

　　The findings are in the journal Science.

　　Of course iron-rich enamel comes with an unfortunate side effect: reddish-brown teeth.

　　But Joester says future human dental treatments that employ iron might find a way around that.

　　"We have the entire periodic table to play with minus a few things that are not too healthy.

　　So I'm sure we can come up with a way to do what the beaver does but do it better and do it in a way that still maintains a nice smile."