If you drink a litre of cold water each day (say 2 almost full pint glasses), then your body expends energy to raise the temperature of this liquid to 37 degrees centigrade. 1 Calorie is the amount of energy required to raise a litre of water by 1 degree, so if we assume that tap water is at 8 degrees (as a year round average) then (37-8) = 29 calories of energy will be used by your body each day or 10585 calories in a year. (Purists please forgive me here- I know there are two different units called the calorie, but I'm using the same one throughout to keep things simple).
A gramme of fat gives up 9 calories when burnt, so you can lose 1.176 kilogrammes per year. This might not sound much, but think of it as just under two stones per decade.
Alternatively you could eat an extra 21 Big Macs every year without a second thought.