Scientists use what’s called a calorimeter to measure the number of calories something has. This device burns the food in pure oxygen and then measures the amount of heat given off. This is relatively close to the amount of energy your body gets because with uses the oxygen you breathe to react with the food you eat.
More specifically how a calorimeter works is this;
A calorimeter is a device that you place a small bit of weighed out food (e.g. 10 grams), then fill with pure oxygen and ignite it with a fuse. Then you can measure the rise in temperature of the (measured amount of water) to know exactly how much energy that released, (e.g. 35 calories/10 grams, so 100g of food would be 350 calories.
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