Help?? Serious Question

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I am wondering what determines how many calories a person burns?

Do "fit" folks burn more or less?
Does body fat have anything to do with it?
Is it intensity and length of workout?
Does ones metabolism have a part in this?

Just curious to know.... how it all works. If 2 people do the same excercise and they burn different amounts. How is that figured?
Thank you!:flowerforyou: