a little physics...it takes an additional 540 heat calories (1000 heat calories = 1 food calorie) to evaporate 1 gram of water to a vapor if the water is already at 100C... so for every 2 grams of water you evaporate using your body as a heat source, you are using up a little over a food calorie of energy... there are about 20 drops in a gram (ml) of water