Why does the flame of a candle rise?

Answer

Flames are very complex things. In a candle flame, for example, candle wax is melted in the wick and evaporated by the heat. A number of complex chemical reactions occur that produce water vapour, CO₂, soot and…produce heat. This heat causes further candle wax to evaporate, so that the process continues…until all the candle wax has been used up. The characteristic drop shape is indeed the result of gravity, which causes the warm air to rise, the wick to get fresh oxygen and the flame to flicker. In the space shuttles, where the effect of gravity can be excluded, a flame is spherical.

http://science.nasa.gov/headlines/y2002/21aug_flameballs.htm