A child rests their chin in their hands, watching a single tall candle burn with a bright golden flame on a wooden table, an empty glass jar sitting nearby.

What does fire need to live?

In the open, this candle burns bright for hours. Drop a glass jar over it and something changes. What is the flame really eating?

A flame isn't a thing sitting there. It's fuel and a gas from the air crashing together — and that crash makes the heat and light. Stop feeding it and there's nothing left to burn.

A brown block of wax fuel enters from the left and a blue puff of oxygen enters from the right; they crash in the middle where a gold flame bursts to life, throwing off heat wiggles.

The gas it eats is oxygen — only about one in five bits of the air. The flame gulps it up, so it needs fresh air arriving all the time. Tap the air to find it.

tap the air

A fire needs three things all at once — fuel, heat (a match starts it, then it stays hot as it burns), and oxygen. Picture them as a triangle. Take away any one corner and the flame falls apart.

Three rounded corner tokens — an orange heat token, a brown wax token, and a blue oxygen puff — are joined into a triangle with a small gold flame glowing at its center.

Your turn — feed the flame. A fire is only as strong as its weakest supply. Turn any single tap down and watch.

Big, healthy flame

Fuelplenty
noneplenty
Heathot
coldhot
Oxygenplenty of oxygen
no oxygenplenty of oxygen

Now the big test.

Two identical lit candles on a table: over the left one a glass jar hovers ready to be lowered, while blue fresh-air arrows swirl toward the open candle on the right.

A glass jar is ready to cover the left candle. Imagine lowering it until its rim seals against the table. The other candle stays in the open. What happens in the first moments after sealing?

The jar has just sealed. It contains the air that was inside a moment ago.

What happens just after the jar seals?