system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does wood burn when you heat it up instead of melting?

Not just wood, but many things burn instead of melt, whereas other things melt instead of burn. Can you burn ice? What if you put ice in wood?
gpt
There are two major forces at work here. The first is the chemical bonds themselves. This would be the carbon-carbon and carbon-hydrogen bonds (among others) that hold a single molecule together. The other force is intermolecular forces, which hold a bunch of molecules together into a cohesive unit. As an analogy, chemical bonds are what make Lego pieces hard to break, while intermolecular forces are what make an entire Lego structure hard to break.

When you heat something, you're putting a lot of energy into it, in the form of vibrations. These vibrations strain against both of those two forces.

If the chemical bonds break first, you get combustion. The individual atoms start flying apart and shedding energy as heat before finding something else to bond with (usually oxygen).

If the intermolecular forces are overcome first, then the thing melts. Molecules start sliding against each other, and the entire structure starts to "flow" like a liquid.