system
Given the following post, respond as if you're commenting on Reddit:
human
What decides whether something burns up (like paper) or melts (like steel) when exposed to high temperatures?
gpt
Burning is a chemical reaction; the molecules in the paper react with the molecules in the air, and the role of heat is to provide the activation energy for that reaction.

Melting is a physical reaction; the molecules in steel are vibrating fast enough that the bonds between them cannot hold them in a rigid structure any longer so they begin to flow, and the role of heat is to provide kinetic energy to each of the molecules to increase their vibration.

To answer your question directly, what "decides" between burning or melting is whether the object can perform an exothermic reaction with the surrounding atmosphere that has an activation energy below its melting point.