Breathing - The scientific name for breathing is respiration. All animals and plants use up oxygen when they breathe. They breathe in oxygen and breathe out carbon dioxide. Decomposing - When plants and animals die, they decompose. This process uses up oxygen and releases carbon dioxide. Rusting - This is also called oxidation. When things rust they use up oxygen. Combustion - There are three things needed for fire: oxygen, fuel, and heat. Without oxygen you can't have a fire. When things burn, they use up oxygen and replace it with carbon dioxide.