How to preview mobile variant in responsive site?

I am converting a non-responsive site. All seems to be going well except that when I'm working on the mobile site and choose to preview either a page or the whole site I only seem to have the desktop site offered. Is it not possible to preview the mobile option?

Flip, simply reduce your browser width.

Acorn

Thanks - I'd read that solution in another context and only as I read your reply, I realised it was the browser on which I was looking at the preview!

Thanks very much, sorry for the dimness!