system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When something burns down and turns into ashes, it takes up significantly less space... where does all the stuff go?

I don't know if I am phrasing this to be understandable, but when you burn down something the resulting ash has a significant smaller volume than the object you burned. Why? Where does all the matter go?
gpt
The air.

The carbon and hydrogen in the fuel combine with oxygen to create CO2 and H2O (water). These just go up in the air (CO2 is gas in room temperature, water from fire is still hot so it's steam). Other particles float away on the hot air as smoke.