If a cookie contains 100 calories, anyone who eats it consumes 100 calories.

But the other side of the energy equation isn't that simple. When it comes to burning off calories, people who do the same activity at the same pace for the same amount of time can burn vastly different numbers of calories, depending on their size. For example, if a family of three jogs side-by-side for 30 minutes, the 175-pound father will burn 400 calories, the 130-pound mother will burn 300 calories and the 65-pound child will burn 180 calories.

"Larger people burn more calories than smaller people, particularly with activities like walking or stair climbing where they have to carry their own weight," says Robert McMurray, professor of exercise and sports science at the University of North Carolina at Chapel Hill. Exercisers are often mislead by workout equipment or charts that don't factor in weight when they proclaim how many calories are being burned by an activity, he says. If an exercise machine or chart calculates the calories burned by an "average" 150-pound person, the results would be "vastly inaccurate" for much larger or smaller exercisers.

In addition, leaning on the hand-rails of a stair climber or keeping a "death grip" on the treadmill railing will greatly decrease your caloric expenditure, says exercise physiologist Steve Farrell of the Cooper Institute for Aerobics Research in Dallas. "The machine is assuming that your legs are carrying your entire body weight," he notes. "But if you're supporting yourself on the handles, you're actually burning about 20 to 30 percent fewer calories than the machine indicates."