Plants use water in the process of photosynthesis, (to make food) but they don't need all of it. They use carbon dioxide, the stuff people and animals breath out, and release oxygen for us to breath, then we breath out carbon dioxide again and the process happens again.
Fun fact: Did you know that rain forests make their own weather? Plants not only release oxygen into the air, but tiny droplets of water. (water vapor, and this part of the water cycle is called transpiration) Those droplets condense and form clouds. That's why it rains every day in tropical forests. Cool huh?
