You have to breathe to live. And you can't breathe without lungs. You breathe in and out. Breathing in and breathing out do different things. Your body needs oxygen to have energy. Oxygen is in the air. You breathe in to take the oxygen from the air. Your lungs put it into your blood. Then your bloodstream takes oxygen all over your body. This is why you breathe harder when you run. Running uses more energy. So your body needs more oxygen. Breathing out is important too. This involves carbon dioxide. After oxygen is taken from air, carbon dioxide remains. It is a gas we do not need. We exhale to get it out of our bodies. Trees breathe too. Their breathing does the opposite of our lungs. Trees breathe in carbon dioxide. They breathe out oxygen. So stand next to a tree and take a deep breath.