The short answer is that when we breathe in air, there's more than oxygen in it. About 20% is oxygen, about 80% is nitrogen, and less than one percent is carbon dioxide. But we breathe out more carbon dioxide than we breathe in.
Our cells all need oxygen to stay alive. They use it in the process of breaking down sugar to realese the energy we need. Our lungs bring in air, and the oxygen flows from the air in our lungs into our blood. Then the oxygen flows from the blood into the cells that need it. The cells basically "use up" the oxygen and produce carbon dioxide (CO2). This CO2 flows into the blood, is carried back to the lungs, and breathed out.
The molecules that make up the CO2 and oxygen don't disappear, they just get rearranged.
Sugar (C6H12O6) and 6 oxygen (O2) go into the process and 6 carbon dioxide (CO2) and 6 water (H2O) come out.
Plants use the sun's energy to make sugar out of water and CO2. What do they "breathe out?"
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