Observe a Candle Under a Glass
Place a lit candle in a small dish of water, lower a clear glass over the candle, and watch: the flame burns down the available oxygen, goes out, and then water rises into the glass. This classic demonstration shows oxygen consumption and air pressure in one elegant sequence—and it's one of the oldest science demonstrations in history, performed by scientists centuries ago.
This experiment is adult-supervised throughout. All flame handling is done by the adult; the child observes and describes what they see.
What You'll Need
- A small candle — A birthday candle or tea light. Secure it in the center of a shallow dish with a small ball of clay.
- A clear glass or jar — A mason jar or a clear drinking glass. Must be large enough to lower over the candle with room to spare.
- Food coloring — Add 10 drops to the water for visibility of water movement.
- Water — Enough to fill the dish about ¼ inch deep.
- An adult — For lighting and managing all flame-related steps.
How to Do It
1. Set up the dish and candle. Press the candle into a small ball of modeling clay in the center of the shallow dish. Fill the dish with about ¼ inch of colored water.
2. Light the candle. An adult lights the candle. Children observe from a safe distance.
3. Lower the glass over the candle. Slowly lower the clear glass over the lit candle, letting it rest on the bottom of the dish so water seals the bottom of the glass. The flame should be completely enclosed in the glass dome.
4. Watch and describe. The flame burns brightly for a few seconds, then dims, then goes out. Immediately after the flame goes out, colored water rises visibly up into the glass, reaching a level higher than the water in the surrounding dish.
5. Discuss what happened. "The flame used up the oxygen inside the glass. When the oxygen was gone, the flame went out—fire needs oxygen to burn. When the warm air inside the glass cooled down after the flame went out, it took up less space. The air pressure dropped, and the water was pushed in from outside."
6. Repeat and vary. Try different candle heights, different glass sizes, different water depths. Does the amount of water that rises change? Does a taller glass produce more water rise?
🎓 Skills Your Child Will Develop
- Combustion and Oxygen — Understanding that fire requires oxygen and that a sealed environment has limited oxygen introduces the chemistry of combustion in a directly observable, visually dramatic form.
- Air Pressure — Understanding that the cooling of sealed air after the flame goes out causes a pressure drop that draws water in introduces air pressure as a physical force that can do visible work.
- Scientific Observation — The sequence of events (flame dims, flame goes out, water rises) must be observed in order and connected causally. Noticing and sequencing these events builds the careful observational attention that scientific description requires.
- Safety Science — Understanding why fire goes out in a closed space, and why this means fire needs air to burn, builds the fire safety intuition that has practical, lifelong value.
Tips & Variations
- Multiple candles: Place three candles under the same glass instead of one. Do three flames use oxygen faster? Does more water rise? (Yes—multiple flames consume oxygen faster, producing a more dramatic demonstration.)
- Size variable: Use different glass sizes over the same candle. A small glass exhausts oxygen quickly (flame goes out sooner); a large glass allows longer burning (flame burns longer before going out). This variation tests a physical variable quantitatively.
My Two Cents
The water rising into the glass is the moment that changes everything. Children who watch it happen don't accept the explanation—they insist on watching it again. The rising water is visible, dramatic, and directly caused by invisible physics (air pressure drop). Making invisible forces visible through observable consequences is what good science demonstrations do.
