5 things The Doctor's soon-to-be-revealed biggest secret could be

We keep hearing this question -- Doctor who? But what does that really mean?

With Doctor Who returning to our screens in less than a week and producers insisting that we'll never guess the Doctor's biggest secret and that, when we find out, our jaws will drop, we decided it's time to look over the possibilities.

So what does "Doctor who?" mean, and what might be his big secret? Chances are it'll probably be a mixture of a few of these, but let's lay them all out on the table.