Butterfly effect

Have you ever heard of butterfly effect? It comes from that saying that a single flap of wings of a butterfly can cause a hurricane on the other side of the world. In other words, one tiny thing can have giant repercussions for the entire world. While every decision everyone makes has the potential to cause such effect, most people do not worry about it, nor should they. If everyone was worried about global repercussions of the breakfast they ate this day, nothing would ever get done. But we time-travelers must think about it all the time. When you do something in the present and it changes the world, it’s the natural course of things. But if you go back in time and sneeze, infecting an ancient person with a disease innocuous to you but lethal to him, you prevent all his future children from existing, and all of their children, and so on, changing the entire course of history. An already established history from which you came.