system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some things burn and other things melt?
gpt
Most things will do both, but one is easier than the other.

Melting is a state change from a solid to a liquid. This requires enough energy (heat) to raise the temperature high enough for the current atmospheric pressure. Some materials simply require too much energy or too low of a pressure for this to happen easily in normal life.

Burning is a chemical reaction between fuel and oxygen. It also takes energy to start (heat), but usually releases even more energy which makes the reaction continue. For some materials, this initial energy is too high to happen easily in normal life.

It gets more complicated when your substance isn't a single material. For example, wood. While we generally think of wood as "burning", there are a lot of other materials in a piece of wood that will melt or evaporate while the cellulose (woody part of wood) burns.

So essentially, it comes down to which of the two needs less energy for the current conditions. This is known as the combustion temperature (burning) vs the melting point (melt).

A somewhat familiar example is magnesium: this is a solid metal at room normal temperatures and pressures that is commonly used in firestarters because it is so flammable and easy to burn. However, if you put it in a non-oxygen environment at the same pressures, it melts rather easily.