system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What in the molecular workings of plastic makes it turn yellow after a long time?

I have noticed in older houses lots of plastic that used to be white turns ye yellow. Why is that?
gpt
1. Most plastics are polymers, aka they're made of extremely long repeating chain molecules linked together.
2. Sunlight has enough energy to break chemical bonds over time, and will eventually break these chains into shorter fragments, and break bonds between different chains too.
3. This one's hard to simplify. The property of "being coloured" comes from how a material's electrons are laid out and how they're able to move around. Basically there's a "sweet spot" of molecule size/length that's good at absorbing visible wavelengths of light. When some colours get absorbed, the object appears coloured since not all colours of light are reflected to your eyes any more.

Putting it together: Over time, the light breaks down the plastic until its polymer chains are short enough that they start absorbing light and appear coloured. 

Related to this, for plastics that are initially transparent: the sun breaking down the plastic at a molecular level also causes "disorder" or just general damage throughout the material. What was once a more neatly packed array becomes fragmented and leads to more light being "scattered" rather than reflected or passed through. That's why clear plastics also go opaque as well as discolouring.