Scientists often divided resources into two groups: renewable and non-renewable. When a resource is used, it takes some time to replace it. If the resource can be replaced quickly and easily, it is called renewable. If it cannot be replaced quickly and easily, it is called non-renewable. All fossil fuels are non-renewable resources. Solar energy, air and water are usually called renewable because there is an unlimited supply. However, this definition may change if people are not careful with these resources. The amount of solar energy that reaches the earth depends on the atmosphere, If the atmosphere is polluted, the solar energy that reaches the earth may be dangerous. If humans continue to pollute the air, it will not contain the correct amounts of these gases. Many resources are limited and non-renewable, and many are in danger of pollution. As a consequence, resources must be conserved and the environment protected. Conservation must become an important part of everyones life.