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Make Water Fill an Upside-down Glass!


Your kids will be amazed to see water move up and start to fill an upside-down glass, as you show them the power of air pressure and what creates a vacuum in this fun experiment!


What to get:  

  • A short candle, like a votive candle
  • A glass that is taller than the candle
  • A pie plate
  • Water
  • Liquid food coloring

What You Do: 

  1. Pour water in to the pie plate until it’s about 1/2″ (1cm) deep.
  2. Add a drop of food coloring and stir to distribute the color.
  3. Place the candle in the center of the pie plate and have an adult light it.
  4. When the candle is burning brightly, lower the glass over it with the rim resting evenly on the pie plate.
  5. Watch what happens when the candle goes out!

The science behind the experiment:  

The candle flame warms the air, which expands, but when the candle has no more oxygen to burn it goes out, and the air inside the glass cools and contracts.  Because cool air takes up less space and because no more air can get in, the air inside the glass now has a lower pressure than the air outside of the glass.  This creates a vacuum that will last until the pressure is the same inside and outside the glass.  As the water rises to fill the upside-down glass, it compresses the space available to the air and equalizes the pressure. 

Scientists ask questions:

  1. Will the water rise higher if you use a bigger glass? A bigger candle?
  2. Does the temperature of the water make a difference?
  3. What happens if you use different liquids, like milk or orange juice?