[OAB 67] It is pointed out that the present quantity of oxygen in Earth's atmosphere could be generated by plants in 5,000 years. That may be true, but this One-Sided Equation fails to account for processes that remove oxygen from the atmosphere, most notably the breathing of animals. The balance between plant photosynthesis (which turns carbon dioxide into oxygen) and animal breathing (which turns oxygen into carbon dioxide) has kept the amount of Earth's oxygen in equilibrium for a long time.