They are abrasive and can wear your enamel away and in the long-term lead to sensitivity and darkening of your teeth as the enamel wears and the inner amber-coloured dentine becomes visible.
2
看门人总是面带笑容，喜欢开玩笑，长着一口发黑的牙齿，还总是在他的小屋里打瞌睡。
The always grinning, always joking black-toothed gatekeeper is always sleeping in his small cubby.
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