The law of conservation of mass states no matter is created or destroyed. This means that the matter currently on Earth is the only matter that will ever be available to us. However, it also means that this matter will not be destroyed. So, water, oxygen, carbon, and nitrogen are all used by living things in our world and then recycled to be used again.
One example of this is the water cycle. More water is never added or taken from the world, it is just reused. First, water starts in some body of water and then evaporates into the air. Then, the water falls back to Earth as precipitation.
