So why is fire not a living thing? It breathes oxygen, reproduces, grows, dies, and consumes fuel.

A few of the best/funniest student answers:

"Fire doesn't carry a replication code; i.e. DNA."
"Fire doesn't have skin." (I think the student meant a 'protective covering', which pretty much every living thing on Earth has in some way).
"Fire doesn't require water, and all living things require water."
"Fire is not a living thing because humans can't eat fire."
"Fire is not a living thing because it can't live with other fires."

And the correct answer, which nobody sumbitted in spite of them all asking their parents and homeroom teachers, is: Fire is not a thing. The trick is that when one hears the question, one goes into all the ways in which fire may or may not be living. But whether it is a "thing," properly speaking, goes without consideration.

Fire is simply a process of oxidation. If you have flaming wood, there are only two material things: the wood (as it oxidizes into a new carbon), and the oxygen bonding to it. The fire is just the light (and sound) energy that the process gives off! It's not a thing in itself the way a life form is.