“I don’t understand.”
The following video is a short cut of a lecture by James Randi about PhDs:
Well, I expect my certificate (if I receive it) to come by postal mail (sad but true :-(), but perhaps I should look for Anthrax-Scanners, maybe the scanners pick something up too. 😉
Honestly, never loose your ability to say these two sentences: “I don’t know.” or “I was wrong.”, at least to yourself. Randi is right1, in an academic context, especially in an exam or a public lecture, these sentences will almost never be used by people who came this far. They are simply suicidal in this setting. But if you loose sight of your limitations, the things you do not know, if you confuse your guesses with the things you empirically know, that crucial distinction — if you ever loose yourself in your self-assumed expertise and do not want to find out, you will no longer be creative as a scientist. You might succeed in academia, especially when talking to the public, but you will never find out anything new.
So, why not use another of Randi’s quotes in these situations: “I just don’t know, but I’ll find out.” 🙂
1 Without wanting to nitpick, in academic contexts, it would not be “just a theory” but a hypothesis — a theory is much more than an educated guess.