Why floss? Even the most careful brushing reaches only three-fifths of all tooth surfaces. And dental floss can go where为什么要用牙线？因为即使最认真地刷牙也只能刷到所有牙齿表面的五分之三，而牙线能够到达牙刷从没到过的地方：牙缝和牙龈线附近。