When you stop eating when you are comfortably full and begin some other activity, ask yourself how you feel about you. What is your self-opinion?
If you are like most, being aware of the body's amazing ability to respond to food sensually is very satisfying. And knowing that you have treated your body well--honored your body--throughout the meal or snack helps in developing a culture of self-care that can extend to other areas of your life (exercising in a healthy way, for example).
You may also have a strange psychological reaction to treating yourself well: "I don't deserve this. I'm uncomfortable doing this because it feels good, and I'm not supposed to feel good." And there is a real challenge. It takes a lot of courage to exist outside your comfort zone, whether the "extreme" place is much better than your normal way of being or much worse. Living in a new and better place, and staying there, can be a big adventure.
If you have had difficulty working your way through this exercise, don't worry. It will take some practice to eat with real awareness, particularly in a culture that works so hard to take your attention away from what you are eating, how much you are eating, and how satisfying it is (or isn't). Continuing to eat mindfully will make all those distractions very, very boring!
Sources:
DayOne Publishing. "Mindful Eating." Accessed 2 May 2007.
Tribole, Evelyn and Elyse Resch. Intuitive Eating: A Revolutionary Program That Works. New York: St. Martin's Press, 2003.

