| Mike Miller |
First, we gain or lose weight based on how much we eat compared to how much food we use. Calories are units of energy, much like liters are units of liquid measure. If you eat 500 Calories, then you ate 500 units of energy. Once you eat the Calories, your body needs to put the energy into use. If I fill my car with gas, it burns it in order to run. Meanwhile it stores the gas it hasn't used yet in the gas tank.