calories everywhere

You can find 100 of them in conveniently packaged snacks, our bodies burn them all the time, and yet they can be neither seen nor touched. That is because a calorie is technically a unit of heat. Just like how feet and meters are ways to quantify distance, calories quantify heat. More specifically, a calorie is the amount of heat required to raise the temperature of one gram of water (~1 mL) from 14.5°C to 15.5°C. Another popular unit used to quantify heat is the joule, which is equal to approximately 0.239 calories. When expressed as a unit of mechanical energy, 1 calorie = 4.2 joules. Lastly, a British Thermal Unit (BTU), more famous for measuring the cooling capacity of air conditioners, is the amount of heat required to warm a gallon of water by 1°F. The important point to remember is simply that a calorie is, albeit somewhat abstract, a unit of heat. Knowing that calories are a measure of heat is important, although less useful than knowing about the calories in food, and how they are handled by the body. Calories, whether those in food or those expended by the human body, are measured by a procedure known as calorimetry.