Oxidize means to combine with oxygen, usually in a way that changes how something looks or works. When iron oxidizes in the presence of water and air, it forms rust, that reddish-brown coating that makes old bikes and nails look crusty. When you cut open an apple and leave it on the counter, the exposed flesh turns brown because it's oxidizing.

The process happens constantly around us. Burning wood oxidizes rapidly, releasing heat and light as the wood combines with oxygen in the air. Your body oxidizes the food you eat more slowly and carefully, using oxygen from your breath to release energy your cells can use. A penny turns greenish over time because the copper oxidizes into a different substance.

Oxidation is the noun form, like when someone talks about preventing the oxidation of metal by painting it. Scientists use the word in chemistry to describe reactions where substances lose electrons, but the everyday meaning focuses on the way oxygen transforms materials. When something oxidizes, it's actually changing into a different chemical substance, a transformation that goes deeper than surface dirt or grime.