How does eating less calories result in weight loss?

I'd like to get a clearer picture. If you have 30lbs to lose, how does eating less calories result in a slimmer body overtime? I understand the concept of calories in/burn them for energy but if you already have the extra weight on your body, what is happening as you lose weight?
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Thanks everyone - why does the body tap into the body fat stores if today we ate less? Which body fat is tapped into and why does it go so slowly?
How does eating less calories result in weight loss?
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