茶水放久了，为什么颜色变深？

Tea leaves hold special molecules called tannins. 茶叶里有一种叫茶多酚的东西。 Tannins love to meet oxygen in the air. 茶多酚最爱和空气里的氧气见面。 When they meet, a slow reaction starts. 一见面，就发生慢慢的变化。 The tea turns from light to dark. 茶水从浅色变成深色。 The longer it waits, the darker it gets. 放得越久，颜色就越深。 This change is called oxidation. 这个过程就叫氧化。