Just for fun: take a look at this moving image. Don’t read the accompanying text straight away – first decide whether you think the figure is spinning clockwise or anti-clockwise. (Apparently this will tell you something interesting about your brain use, though I think the illusion is interesting enough regardless of the reliability of this claim.) At first I could only see it spinning clockwise; now I can only see it spinning anti-clockwise. Apparently some people can change what they see at will, but not me.
Update: I have now been able to change at will a couple of times, by focussing on one of the hands and thinking about when it would have to be in front and when behind if the figure were spinning in the opposite direction!