Habit stacking, by definition, is the building of a new life habit by "stacking" the new habit atop a habit which already exists. You see, habits which we currently practice on a regular basis have a strong root system within us. By beginning to build a new habit on a powerful existing foundation, in this case another habit, you are beginning the job with the hard part already completed for you. There is no reason to start building the new habit from the ground up. The premise of habit stacking is quite simple.