You have to breathe in order to stay alive. When you breathe in, or inhale, you draw air, which contains a gas called oxygen, into your lungs. You cannot live without oxygen, for oxygen changes the food you have eaten into energy. Your body uses this energy to keep you warm, to make new cells, to move your muscles, and to send messages along your nerves. You need all this energy to stay alive.
You can actually feel this manufactured energy after you have eaten a heavy meal, for it is then that you feel warm. This is because when the food is changed into energy, heat is given off. This heat helps keep the inside of your body warm even if the outside temperature is zero. You cannot stay alive if the temperature in your body falls too low.