Natural resources are of two kinds. They are renewable and non-renewable. Resources which are replaced by natural processes are called renewable. Air, water and sunlight are renewable. They do not get over when we use them because nature replaces them. we use oxygen of the air, for example. And the natural process of photosynthesis replaces it. Non-renewable resources, on the other hand, are those which cannot be replaced easily by the natural process, we run the risk of exhausting them if we use them up too fast. Fossil fuels, for example, took millions of years to form. If we over utilize them, we may run out of them because nature cannot replace them that fast.