Calorie conscious looks for calories they are getting from the food they eat. It is always some positive number and it is suggested one should go to low calorie food that is nutritious as well.
But, do you know that there exist food with negative calories. Surprisingly, they do exist. These food have positive calories but you burn even more calories to digest them. So, net effect is that they have negative calories.
Take example of an apple. A medium-sized apple has 50 calories but you require 65 calories to digest it.
You also know that apple is quite nutritious and a source of fibre too. Thats why docs say "An apple a day, keeps doctors away."
There are many other negative calorie foods. Just google the net to find them and you will burn few more calories in that too...