When do you really know?

Perhaps more on E8 and Garret Lisi later. By far my favorite thoughts for this weekend.

When do you know?
You do not really understand something unless you can explain it to your grandmother. – Albert Einstein

When do you really know?
“You do not really understand something unless you can explain it to a string theorist.” [comments link]

When can you say what you know?
The mark of a great scientific idea is a complete lack of any comprehensible relation to what many others have successfully done before. [link]