There is one thing that each of us is more familiar with than anyone else in the world, and that is our own language and the way we speak. Most of us are accustomed to using different language when talking to parents, children, spouses and partners, co-workers, friends, etc. How do we know when to switch gears? When we hear a new word, how do we know when we should incorporate it? Or do we reject it? How do all these different streams come together in a population, and what drives changes in language as time goes on?