What keeps a little candle flame alive?
Look at the little flame… it dances and dances. A flame is hungry — it eats the air all around it, just like you eat your dinner. Now watch… a big glass jar comes down over the candle, and traps the air inside. Ready to guess? Will the flame keep dancing… or go to sleep?
After you watchWhat keeps a little candle flame alive?
The short answer
A flame needs air to stay alive. A candle eats the air around it, so out in the open where fresh air keeps coming, it dances and dances. Trap it under a glass jar and the air inside runs out, so the flame goes to sleep — even with lots of candle left.
Try this next
- What if you used a giant jar instead of a little one? Guess whether the flame sleeps faster or slower, then imagine the bigger jar holding more air for the hungry flame to eat.
- What if you lifted the jar up a tiny bit? Guess whether a sip of fresh air is enough to wake the flame back up, then watch it come alive in the explainer.
The whole story
How it works
A flame is like a hungry little eater: it uses up the air right around it to keep burning. In the open, new air keeps flowing in from every side, so the flame is always fed. Under a closed glass jar there is only a little air, and once the flame eats it all up there is no more, so it goes out. Lift the jar and fresh air rushes back in, and the flame can come alive again.
What people get wrong
Lots of little kids think covering a fire keeps it safe, or that it goes out because the candle is used up. But the covered candle still has plenty of candle left — what it ran out of was air. Take the air away and the flame cannot live.
The catch
Putting a lid over a flame is exactly how grown-ups put out a small fire safely — cover it and the air runs out. But the very same air a flame needs is also why you should never play near fire: out in the open, air keeps it alive and it can spread.
Questions kids ask
Why does the candle go out under the glass?
Because the air inside the jar runs out. A flame eats the air to stay alive, and a closed jar only holds a little. Once that air is used up, the flame goes to sleep — even though there is still lots of candle left.
Does the candle run out of wax under the jar?
No. When a covered candle goes out it still has almost all of its wax. It did not run out of candle — it ran out of air.
What does a fire need to live?
A fire needs air. It also needs something to burn and heat to keep it going, but air is the one a kid can take away with a glass jar to watch the flame go out.
Talk about it
- We put a jar over the candle. What do you think the flame will do — and what did it run out of?
- There's still lots of candle left, but the flame went out. So what does fire need to live?
- When we blow out birthday candles, are we giving the flame more air or taking its air away?
For grown-ups
Burning is a chemical reaction that needs oxygen from the air, along with fuel and heat. Under a sealed jar the flame uses up the trapped oxygen and goes out long before the wax is gone — it ran out of air, not fuel. Smothering a small flame with a lid works by cutting off that oxygen.
Keep going
What else makes you wonder?
- If you used a really BIG jar, would the flame stay awake longer before it sleeps?
- When you blow out a birthday candle, where does the little flame go?
- A flame eats the air — what other hungry things can you not even see eating?