system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does plastic Tupperware take on food stains after a while?

Normally I see this with acidic foods, usually tomato based pasta sauce.
gpt
Plastic is porous.  Meaning, at the micro level (think zoom in with a microscope), the surface of plastic is actually very bumpy, with many gaps.

Stuff like tomato gets embedded into these bumps.

If you microwave your tupperware, it affects the plastic itself (feel how it is easier to bend when warmed in the microwave?).  The plastic's porous surface actually expands a little.  This makes it even easier for the tomato to embed into the plastic.  The tupperware then cools, and shrinks back in... trapping the tomato permanently.