Nothing is more obvious about the nature of time than the fact that the past is different from the future. Physics ascribes this difference to the fact that entropy -- randomness or disorder -- increases with time. I will talk about why this is so, a mystery that will lead us to the origin of the universe. I'll also talk about why the increase of entropy is responsible for all of the differences between past and future, from memory to aging. Finally, I'll talk about the relationship between entropy and complexity, and why it's not so surprising that complex life came into existence in a decaying universe.