The scale. It can be a powerful ally in the quest to maintain a healthy body -- or it can be an unhealthy enabler that feeds into an obsession about your weight.
Check it at the same time of day once a week and it can help you gauge how different levels of activity and calorie consumption affect your body weight. Check it more than once a day and it could be a sign that you're too fixated on your weight. Not only is compulsive weighing bad because of the stress it'll add to your day, it's also important to realize that body weight naturally fluctuates throughout the day, so multiple weigh-ins don't really tell you much about true weight loss or gain.
Another sign that you're "scale obsessed" is that the little number you see in its window feels more like a judgment than a piece of information -- and that number can impact how you feel about yourself throughout the day.
In combating the siren song of the scale, it's important to remember that it is just one tool that helps you determine how healthy you are. The way your clothes fit, the amount of energy you have and the quality of the foods you put in your mouth all play an equally important role in determining your overall condition.