The Idea

We tend to associate secure passwords with complicated and hard-to-remember passwords. But it doesn't have to be this way.

We make password difficult to guess by increasing entropy — the degree of uncertainty in the password. The higher the entropy, the harder it is to guess the password.

One way of increasing entropy is to make passwords more complicated. Another way is to make the password longer, but keeping it simple, as the following xkcd comic shows:

While using a single dictionary as a password is horribly insecure and can be guessed in seconds, guessing multiple words gets exponentially harder. Using five words should be able to stop any attempts you can be expect, while eight words can probably deter even the most determined attackers for the next ten years or so.