Arrow of time

The arrow of time is a way of looking at the world that talks about why things seem to happen in a certain order. For example, a glass might break, but it will never put itself back together. That's because the arrow of time points in one direction - from the past to the present to the future. It means that things usually move forward in time, but never backward. It also means that some things happen once and never again - like a glass breaking!