"What is a flame?"
I clearly remember asking my dad this very question, and (like Mr. Alda) not getting an answer that made me feel any closer to understanding. I don't know why, but I remember exactly where we were and what time of day it was, walking to the grocery store after dinner one evening. Maybe it's so clear in my mind because the event made me realize my dad couldn't necessarily answer all my questions.
Mr. Alda's challenge (which you should try; if you do, feel free to put it in comments or link to it) is to come up with an answer that will satisfy an 11-year old. So, here's my go--this is off the cuff, in the spirit of me springing the question on my dad, so no editing:
What is a flame? It's kind of hard to define, because it's two things at once--it's a thing and a process, the same way a dance is a thing and a process. I could say a dance is a thing, a set of steps that a group of people do, but it's also a process, where the people are doing a set of steps and moving around from start to finish.
So, a flame's like that--it's a thing, a chemical reaction involving some atoms changing partners. But it's also a process, that starts with some wood and some air and ends up with some ash and some smoke. The process is really neat.
If you're burning wood, it's got a bunch of atoms of carbon that are all bound to each other. Those atoms are OK with their situation, but they'd much rather be bound to oxygen to make carbon dioxide. But if they're in wood, it's really hard for them to do that. The first thing that's got to happen is that you have to give those carbon atoms in the wood the chance to react--get them on the dance floor, like--and the way you do that is with a lot of heat, like from a match. That actually makes wood vapor, kind of like the way that water makes water vapor or steam when you heat it up. Word! Wood--as a gas!
Anyway, wood gas is on the dance floor and now it can bump into oxygen and react with it. That reaction releases a bunch of energy--energy that used to be needed to hold carbon atoms together but isn't needed any more--and we see some of that energy as light, feel some of it as heat, and some more of that energy is used to vaporize more wood. So, the "thing" of a flame is that chemical reaction, but the "process" of a flame is wood or wax getting heated up, vaporizing so it can react with oxygen, reacting with oxygen, giving up a lot of energy, and using some of that energy to keep the process going. You're left with some carbon dioxide, and some ash, which is the bit of wood that can't get vaporized to burn.
I love looking at fire--I mean, it's toasty warm and makes neat patterns, but it's so cool to think about those carbon atoms getting heated up to get on the dance floor, join with oxygen, and keep the dance going.
Well, there it is, no editing. It's definitely not perfect, and a couple of bits make me cringe. Not sure if I should enter, edit and enter, or just give it a miss. I wonder if 11-year old me would be satisfied. As I recall, when I asked my dad this question, I had just finished reading a YA-science book about plasma and how wonderful the world would be when we have fusion power, and my dad was a professor of biochemistry. There's no doubt I wouldn't be able to deliver the above explanation to 11-year old me--I doubt I'd be able to refrain from interrupting with a question.