system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes white things turn a dark yellow as they get older?
gpt
There are two main groups of things which go through this transformation for different reasons.

The first are things which have had a blue additive.  This is commonly done in white paints, clothing, and plastics.  It turns out that pure white doesn't actually look white to us when it is outside in the sun.  To make things look white you want to make them be a little more blue, so the additive makes them look more white to our eyes.  Overtime these additives breakdown and so the items return to true white, but this looks slightly yellow to us.

The other is paper products.  Paper products are bleached to make them look white, but if not treated properly there is a major component in paper that breaks down overtime into acid.  This acid eats the paper slightly changing its color.