Old habits die hard, and old passwords die harder. But if you ever needed extra motivation to forget that ancient password you've been reusing for years, you'll find it in Yahoo's recent admittance that a 2013 security breach affected all 3 billion user accounts on the site. Think about that number. That's 3 billion passwords. There weren't 3 billion people on the internet in 2013, but there were that many Yahoo accounts because some people had several Yahoo accounts at the time.