What is a Calorie?

We have become much more conscious of food labels and read the back of the pack, focusing especially on one number: calories per serving. The calorie is actually a unit of energy. The energy in food is measured and reported in kilocalories (what we mistakenly call "calories"). The average male requires 2,000–2,500 kcal per day. But what does this number even mean?

First off, 1 kilocalorie = 1,000 calories (duh for non-US readers).

• 100 kilocalories, i.e., a very small snack, is enough to charge your phone from 0 to 100% EIGHT times. Phones are so energy-efficient.
• 2,000 kilocalories, i.e., a day of eating, is enough to bring 20 litres of ice-cold water to a boil (but not vaporize it, because latent heat of vaporization is a thing).
• 15,000 kilocalories, i.e., what you eat in a week, is enough to put a 1 kg satellite into low-Earth orbit, with some energy left over.

The human body, on average, has about 100,000 kcal of stored energy (mostly in your liver)—50 days of "reserve power." It is pretty good at converting chemical energy into mechanical energy.