Every time you run, think, or even sleep, your body is burning fuel — and that fuel comes from food. Scientists measure the energy in food in units called calories. A single day of ordinary living costs a child a couple of thousand of them. A slice of bread, a bowl of soup, a bar of chocolate: each carries a certain number of calories your body can burn to keep warm, to move, and to grow. Chocolate happens to be packed with energy, which is one reason a hungry child treasures it so much. Fats and sugars carry more energy than almost anything else, which is why a small bar can hold more than a large plate of vegetables.

When a person does not eat enough, the body has too little fuel to work with, and bit by bit it begins to weaken. The body protects the brain and the heart first, and everything else is asked to make do. A starving person feels cold, because there is not enough energy to stay warm; they feel tired, because every movement costs fuel they cannot spare; and they slowly grow thin as the body uses up its own stores. Cold weather makes the shortage worse, because staying warm burns fuel a hungry body does not have. This is why a long, hungry winter is so dangerous — and why a single good meal can feel, to someone who has gone without, almost like a miracle.

No right or wrong — close the screen and tell someone nearby what happened in this part. Can’t find anyone? Tell yourself out loud. The moment you teach it, you truly understand it.