system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does fire destroy stuff?

This is hard to properly express my question, but what does fire do to destroy things? Like how does it burn a whole house down leaving ashes on the ground, and why aren't the ashes destroyed? Probably some obvious answer I'm overlooking but it is what it is.
gpt
Fire is the process of transforming something chemically into something else. It usually happens in two stages. First, for complex stuff like wood or paper, you take something and heat it up, and that heat destroys the chemical bonds in that substance. The big molecules in it that are very long and solid and lock its parts into place break into smaller molecules. So cellulose cracks into gases like methane (a very light form of gaseous fuel) and leftover stuff like carbon (which is what makes char black), and the wood appears to disintegrate.

Then the heat causes the gasses to combine with oxygen and combust - change into carbon dioxide + water molecules - and give off more heat, making a self-sustaining "fire". When they're gone, it works on the little bit of carbon that's left to turn that into gas - carbon dioxide - too.

What's left is a tiny amount of solids that are made of molecules in traces of wood that don't turn into gas when combined with oxygen (for example, potassium), stuff that's already oxidized (for example, dust from sand - silicon dioxide - blown into that wood from a nearby beach) and stuff that doesn't burn at the temperature of the fire (for example, the copper piping in your house).