Air Pressure: Candle in a Jar

Use a chunk of clay to hold a candle in the bottom of a glass jar (or other container, as seen above).

Pour about an inch or 2 of water into the jar.

Light the candle and place another glass jar (smaller than the first, so it fits inside) upside down over the candle.

What happens:
The fire is extinguished and the water rises.

Why:
The candle burns up the oxygen. Once the oxygen molecules have been consumed, the remaining air takes up less space, so the air outside of the jar pushes the water up.

Some tips:
1 - Make sure you don't set the inverted jar down on the bottom of the larger jar, hold it off the bottom - it will create a seal and the water won't rise!

2 - Colored water may make it easier to see the water level rise - as you can see above, regular, colorless water is difficult to see (it was in person too, not just in the picture).