氧化是一种化学变化，东西和空气里的氧气结合就会氧化。

Oxidation is a chemical change where a substance combines with oxygen in the air.

比如铁放久了会生锈，苹果切开变黄，这些都是氧化。

For example, iron rusts after a long time, and apples turn yellow when cut; these are all oxidation.