I'm stumped

I was watching It's Academic, a local TV quiz show
among area high school teams, and one of the questions
was:

What is the area of this square?
Given: On the monitor, a square with the length of the diagonal shown as 12.

The kid did it in his head in about 5 seconds.

It has something to do with .707 and the square-root of 2.
but I've developed brain-lock and don't get it.


Spoo