system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does clear phone cases turn yellowish over time?
gpt
Think of the material as being made of long strings. Something like a bug screen on a window, in fact a whole bunch of those screens stacked up so the holes match up perfectly and light can keep passing through. 

Over time the environment (mostly UV light) breaks some of those strings down. As they break into smaller units, the various twists and snags start to block some of the grid of openings, so instead of all the light passing straight through some of it is blocked and reflected back as that yellowish color. 

We're talking about polymers breaking down, here, but maybe that visual makes sense? Polymers are long strings of molecules arranged just so into a material. As they break down into smaller, more randomly arranged strings their appearance changes and they can go through physical property changes like becoming brittle, depending on the exact material on question. As the structure breaks down, other additives can also leak into bits of the structure it wasn't meant to be in and colors it, too.