When it comes to so-called good foods and bad foods, it's pretty easy to separate a green salad from a piece of pie. But some healthy foods can become less beneficial for you simply because of the way you cook them.

Researchers analyzed three years of eating patterns of kids between the ages of 7 and 13 who gained excess weight in that time, and identified the foods most likely to get the blame.

Fat-based spreads like butter, desserts, candy and sugary beverages and processed meats were on the list. But so were poultry and fish when breaded and battered, and potatoes cooked in oil -- from French fries to chips. In these cases, the cooking methods undermined the value of otherwise healthful foods, and not just the chicken and fish.

The researchers pointed out that when boiled or mashed without any fat, potatoes are satisfying, yet not associated with unwanted weight gain. Of course, if you leave off the coatings, chicken and fish won't lead to excess pounds either. Note: Baking, poached and light sauteing are tasty alternatives, especially when you add herbs to increase flavor.

The researchers also singled out whole grains and high-fiber cereals as good foods that don'tpromote overweight. These happen to also be high in fiber, which is important for children as well as adults -- and many kids don't get enough.

The bottom line: prepare healthy foods in healthy ways so that kids -- as well as mom and dad -- will get vital nutrients without unwanted calories that come from cooking techniques like breading and deep frying.