Losing Air

Explanation

When you burn a candle you are reacting oxygen from the air with carbon and hydrogen in the candle. This will reduce the amount of oxygen in the glass, but it will also increase the amount of carbon-dioxide in the glass.

Overall 3 molecules of oxygen will produce two of water which condense and two of carbon-dioxide. So the volume of gas should reduce by 1/3 of the oxygen burnt. Oxygen only makes up 20% of the air, and a candle will stop burning when the level of oxygen drops to about 15%, so the gas should shrink by about 1/3 of 5% so about 1-2%. This is probably too small an effect for you to notice.

What is actually going on?

Most of the effect is to do with the temperature of the gasses. As the candle burns it produces a plume of hot gasses rising above it., when you put the glass over it traps these hot gasses.

There is a plume of hot air and combustion products above a candle	The glass traps the hot gasses

The glass slowly fills up with the hot air which takes up more space than cold air so some air escapes around the edges of the glass. Eventually the candle has used enough oxygen to stop itself burning, and it goes out.

The air then starts to cool, and as gasses cool they shrink so it pulls water into the glass producing a much larger effect than just the loss of oxygen.

The glass fills with hot air with reduced oxygen levels and puts out the candle	The air then cools and shrinks pulling water into the glass