system
Given the following post, respond as if you're commenting on Reddit:
human
Why do substances melt when heated while others solidify?

Eggs solidify when heated, cheese melts. Butter melts. Some substances can reliquify or resolidify but e.g. a solidified egg will stay solid.

Why is that?
gpt
When cooking eggs, you are denaturing long protein chains that are folded and curled up on themselves.  These separate 'balls' of protein suspended as a colloid in the waterborne medium of the egg acts as a liquid.  When heated, they no longer fold in on themselves and form a mat of crosslinked polymers which acts as a solid.  It is not a phase-change, like melting or freezing is.