So if animals went around eating and burning sugar all the time and there was no way to get this oxygen back into the air, then we would all run out of oxygen and die. Here is where trees come in again. Trees and plants have this all worked out. Sugar really is the key to everything that happens on this planet. Here's how it works.
Plants make sugar using sunlight.
We burn the sugar and release the energy from the sunlight stored in the sugar. I know it is hard to believe that when you burn sugar you actually make water, but it's true. Water seems like something that is always water, but it's not. Sometimes it's part of sugar. Ok, we still need to find some way that eating all this sugar and burning it up with oxygen is not going to use up all the oxygen.