system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do color changing cups work?

I have a cup that changes from white to purple when cold. How does that happen?
gpt
They are coated with thermochromic paint. The paint can change colour or become transparent when subjected to heat (or by extension cold). How these paints change colour is a little bit beyond eli5 imo but still in short, heat changes the structure of the paint temporarily allowing it to absorb/reflect different wavelength of lights.