It may superficially sound like a stupid question with a simple answer, but let me explain in more depth. Calories are units of energy, as you probably know, and grams are the the weight of the food. So calories are not directly related to the weight of a food. Foods range in the amount of calories per gram. So the meat of my question is basically this: if you eat the same weight of food, but foods with higher calories why do you start gaining weight? How or why does the energy cause weight gain? This is a possible solution I can think of: the extra calories somehow end up retaining more weight of the food than the otherwise would if you had consumed lower calorie foods. As an extra thought experiment related to this, imagine a "super seed" of 3,000 calories but only a weight of 1 gram. How could calories cause weight gain then? Obviously this is impossible, but it's just a thought experiment to view an extreme form of the question. Thanks for anyone who replies!