What happens to the big balloon when we light a warm flame under it?
Look… a great big balloon, sitting on the grass. There is a little flame down underneath it. In a moment, we are going to light that warm flame. What do you think the big balloon will do? Will it float up, up, up… or will it stay down on the ground? Ready to guess?
After you watchWhat happens to the big balloon when we light a warm flame under it?
The short answer
When you light a flame under a hot-air balloon, the balloon floats up. Heating the air inside makes it lighter than the cool air around it, and lighter things float up.
Try this next
- What if we keep the flame on for a long, long time? Light the flame and keep watching. Guess first: does the balloon keep floating higher, or stop and hover?
- What if there were no flame at all, ever? Picture the balloon with a flame that never lights. Guess first: would a balloon full of plain cool air ever lift off the grass?
The whole story
How it works
A flame warms the air trapped inside the balloon. Warm air is lighter than the cool air all around it, so the whole balloon becomes lighter than the air it sits in and rises off the ground. When the flame goes out, the air cools, gets heavier again, and the balloon drifts back down.
What people get wrong
Little kids often think the flame pushes the balloon up like a rocket, or that the balloon is full of something special. It is just plain air — warming it is what makes it light enough to float, and the flame does not shove it upward at all.
The catch
Warm air floats up, which is the fun part, but warm air cools off fast. Once it cools it gets heavy again and the balloon sinks, so the flame has to be lit again and again to keep it up. Cool air is heavy and stays low and steady, but it will never lift the balloon.
Questions kids ask
Why does warm air float up?
Warm air is lighter than cool air. When something is lighter than the air around it, it floats up — just like a bubble floats up in water.
Why does the balloon come back down?
When the flame goes out, the air inside cools off. Cool air is heavier, so the balloon gets heavier too and floats gently back down to the ground.
Does the flame push the balloon up?
No. The flame does not push it like a rocket. The flame just warms the air, and the warm, light air is what carries the balloon up.
Talk about it
- Before we light it, what do you think will happen to the balloon?
- Where in our house do you think the warm air goes — up high, or down low?
- Have you ever felt warm air rising, like over a heater or a candle?
For grown-ups
Heating the air inside the envelope makes it expand and become less dense (fewer molecules in the same volume). A balloon of lower-density warm air weighs less than the equal volume of cooler, denser air it displaces, so the upward buoyant force beats its weight and it rises — Archimedes' principle with air as the fluid. For ages 4–6 we keep it to one sensory idea: warm air is light, and light things float up.
Keep going
What else makes you wonder?
- Where do you think the balloon stops going up?
- Warm air floats up — what else have you seen float up, like a bubble or a feather?
- If warm air goes up, where do you think the cool air goes?