Why does a candle flame die under a glass but burn forever in the open?
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TPWDiscover | What is the Fire Triangle?
A park ranger removes oxygen, heat, and fuel one at a time so the fire triangle becomes visible.
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What else makes you wonder?
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More trapped air buys more seconds, but it's still a fixed amount. What would have to be true about the jar for the air to never run out?
After you watchWhy does a candle flame die under a glass but burn forever in the open?
The short answer
A flame needs fuel, an oxidizer and sufficient heat. A sealed candle can keep burning briefly using air already inside; then falling oxygen and accumulating combustion products make conditions unable to sustain the flame. Oxygen and wax can remain when it goes out.
Try this next
- What if you put a tall, skinny glass and a short, wide glass over two flames at the same time? Predict which one stays lit longer before you drop them, then watch the trapped air drain in each — you're testing how much oxygen the glass holds, not the wax.
- What if you lifted the glass a tiny bit every few seconds? Guess whether the flame survives, then try cracking the seal in the explainer or at home — see if a sip of fresh air is enough to keep the reaction fed.
The whole story
How it works
The jar seals at timeline value 40. The left flame remains full through 55, then fades by 95 while the illustrative oxygen bar falls from 100% to 70% of its starting amount. The endpoint is qualitative, not a universal oxygen threshold or measured timescale. The right candle stays open to fresh air.
What people get wrong
Sealing a jar does not instantly remove its air, and flame extinction does not mean every oxygen molecule was consumed.
The catch
Cutting off the air is exactly how you put out a small fire safely: a pot lid, a fire blanket, or extinguisher foam all smother a flame by blocking its oxygen. But smothering only works when you can cover the fire quickly, so it fails on a large fire that is far too big to seal. Big fires are usually fought with water instead, which works by removing the heat. Blowing on a fire can even backfire, because a gentle puff brings more oxygen and feeds the flame.
Questions kids ask
Does the flame use all oxygen before going out?
Sealing a jar does not instantly remove its air, and flame extinction does not mean every oxygen molecule was consumed.
Talk about it
- Before advancing time, predict what happens immediately after sealing.
- What remains in the picture after the flame goes out?
For grown-ups
The jar seals at timeline value 40. The left flame remains full through 55, then fades by 95 while the illustrative oxygen bar falls from 100% to 70% of its starting amount. The endpoint is qualitative, not a universal oxygen threshold or measured timescale. The right candle stays open to fresh air.