Examples taken from daily experiences are used to introduce the concept of entropy in an intuitive manner, before it is defined in a more formal way. It is shown that the entropy increase due to irreversible transformations (or "unrecoverable" energy) simultaneously determines the level of fresh energy supplies of our society and the damage that it causes to the environment. Minimizing the rate of entropy increase with advanced technologies and society organizations, and keeping it in check with appropriate energy sources, is the key to a sustainable development.