A food calorie (kilocalorie) is the amount of energy required to heat one liter of water one degree C. Therefore, when you drink one liter of ice water your body has to warm it up to 37C, giving it an effective calorie content of negative 37.
Room temperature (25 degree C) water, on the other hand, has only -12 calories per liter.
More collected Geek Thoughts at http://geekthoughts.info.