Quite simple math, since a calorie is the amount of energy required to heat a mL of water by 1°C. Thus a food Calorie (1000 calories) is the amount of energy necessary to heat a liter of water by 1°C. Human body temperature is 37°C, so you burn 37 calories for every liter of 0°C water ingested. The average donut has 195 Calories. Thus you must drink 5.27 liters of 0°C water to cancel out the calories gained from eating a donut. Not very realistic.