Possible Duplicate:
$i^2$ why is it $-1$ when you can show it is $1$?
I was thinking on the following line of thoughts: $1 = \sqrt{1} = \sqrt{-1 \cdot -1} = \sqrt{-1} \cdot \sqrt{-1} = i^2 = -1$
Of course this is not true, but I was wondering which step in this 'line of thoughts' is forbidden to make?
Thanks for the explanation.