If you’ve ever stumbled across old newspapers, you might have noticed an interesting trend: Most of them are yellow—and no, they weren’t always that way. In short, this color-changing phenomenon is due to the process of oxidation. (In other words, the same process that turns a half-eaten apple brown.)

To break down this scientific process, let’s start with the basics. Paper is made from wood, and wood contains two substances called cellulose and lignin. When the chromophores (molecules responsible for color) found in lignin are exposed to air and sunlight, the lignin’s molecules become less stable and absorb a greater amount of light. In turn, the newspaper’s hue gradually darkens from white to yellow.

The reason newspapers are more prone to this color change, in comparison with other types of papers, is because the wood’s lignin isn’t typically filtered out. This both saves on the costs of mass daily printing and leaves the papers more prone to oxidation. (A chemical process called conservation bleaching can be used to produce yellow-resistant papers to avoid discoloration over time.)

Fortunately, the lifespan of newspapers can be lengthened if they’re stored properly. The best way to preserve newspapers is to keep them out of sunlight and store them in acid-free and buffered boxes. Doing this allows the acids released by the papers to be neutralized by the alkaline buffers, helping the papers retain their white color.