You've probably noticed that on the side of your cereal box or milk carton, there's a big table of nutrition facts. In this table, it shows the quantities of vitamins, fat or sodium, but most importantly, it shows how many Calories the food has per serving. You've heard about Calories before, and know that you gain weight if you consume a lot of it, but probably don't know exactly where the measurement comes from.
A dietary Calorie is always spelled with a capital "C" while a physics calorie is always spelled with a lowercase "c". It is very important to not get these mixed up, because as confusing as it may be, you can eat so many more calories and stay healthy than you would if you ate the same amount in Calories.
Specifically, 1,000 physics calories, or "gram calories", is equal to 1 dietary Calorie, or "kilocalorie."
It's not like you'll encounter calories nearly as much as you'll encounter Calories, especially since everything related to diets and health are measured in Calories. Either way, it's still interesting to know where they come from.
1 physics calorie is the amount of energy it takes to heat up 1 gram of water by 1 degree Celsius. Not surprisingly, 1 dietary Calorie is the amount of energy it takes to heat up 1 kilogram of water by 1 degree Celsius. It makes a lot of sense that food is measured in Calories, since otherwise you'd look at the nutrition facts of a single Reese's Peanut Butter Cup and realize that it's 105,000 calories. You probably wouldn't eat that much candy if you saw that, so they crunched the numbers down to make it a little less overwhelming. Food companies do like to trick you, however: they make the "serving size" really small so that their food doesn't appear as bad as it actually is. Next time you're buying cereal, even if you're like me and don't care about the dietary facts, just look at the nutrition table and see how small the serving sizes are. I've seen a box say that 1/2 of a cup of cereal is a serving for 1 person. Think about that the next time you eat 4 bowls of cereal in a row.