Cells in the human skin produce a substance called melanin. When sunlight hits the skin, more melanin is produced. Melanin darkens the skin and is the cause of suntan. Most people tan evenly all over the body, but some people tan just in spots. These little islands of suntan are the cause of freckles. 人类皮肤里的细胞产生一种物质叫黑色素。当阳光照到皮肤，会产生更多的黑色素。黑色素使皮肤变黑是人被晒黑的原因。大多数人是整个身体均匀的晒黑，可一些人是局部晒黑。这些小黑斑就形成了雀斑。 编辑推荐：2009年下半年公共英语考试报名汇总 名师指导公共英语阅读考试技巧 公共英语pets三级全真模拟试卷 公共英语pets二级语法讲解汇总