英语时差：嘴唇为何会干？

Let’s start by reviewing the ways in which the surface of our lips is different from the surface of the skin on the rest of our bodies. Basically, our lips are made of the same mucus membrane that coats the inside of our mouths. This means two things. First, our lips don’t have the same protective outer layer as the skin elsewhere on our bodies. This layer is part of the barrier to evaporation, and its absence means our lip skin retains less fluid. Second, our lips don’t have the same glands and pigments as ordinary skin. For example, lips don’t have oil glands, which also help keep moisture from evaporating, and they contain less melanin, the pigment that helps protect our skin from sunburn.