system
Given the following post, respond as if you're commenting on Reddit:
human
Why does paper turn yellow as it ages?
gpt
It eventually turns paper yellow because of oxidation. Paper is made from wood, which is made up mainly of white cellulose. Wood also has a lot of a dark substance in it called lignin, which ends up in the paper, too, along with the cellulose. The exposure of lignin to air and sunlight is what turns paper yellow.