system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does paper turn yellow and brittle as it ages?
gpt
Wood pulp from trees is made up of two major substances: lignin and cellulose. To make paper, we want just the cellulose part. So ordinary papermaking uses a strong acid, sulfuric acid, to break down the lignin in a sort of "cooking" process. They mostly neutralize the mash before making paper out of the remaining cellulose, but not entirely.

So over time, the leftover acidity in the paper gradually attacks it, breaking down the cellulose (which is much more resistant than lignin, but still, it's acid) and causing that sort of aging.

For long-term survival, that's why people use "acid-free" paper. It costs more but the papermakers use extra care to fully neutralize it. Whereas something meant to be thrown away quickly, like newspaper, uses the cheaper and still very acidic paper. It can start yellowing a day after it's printed.