Balancing how many calories you take in to how many calories you burn off.
For Example: Say I burn 1,500 calories a day on involuntary movement, and I am a moderately active person so i burn 500 calories of walking the halls of school and climbing stairs and things like that. Then how many calories I take in would have to be pretty close to that. So around 2,000 calories is the best intake for that example.