Sidebar on top on responsive view

Hi,

I have managed to move the sidebar to the left on normal view by changing the #primary and #secondary floats.
But when seeing at smaller screen (max-width: 680px) the sidebar (#secondary) is still at the bottom (after) the main content (#primary).
What I want is to put the sidebar on top of the main content.
What css should I change to do this?
Thank you