how does calorimeter work??

A calorimeter works by preforming some reaction (Usualy combustion) in a sealed container which is contained in a larger vessel filled with some liquid of a known specific heat (usually water). When the reaction is preformed, it can either give off heat or absorbe heat to the surroundings (the water). By measuring the temperature change in the water, we can calculate the amount of energy absorbed/released by the substance during the chemical reaction.

http://en.wikipedia.org/wiki/Calorimeter
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/calor.html

Exactly right! I hope you don';t have problems on it, those are sooo painful