If you are near to this picture, Mr Angry is on the left and Mrs Calm is on the right. If you view it from a distance, they switch places!
Option 1. Just look at this picture on your computer screen and vary the distance from which you look at it. For some people, even squinting or narrowing your eyes will work.
Option 2. Using the 'Save As' option (or equivalent) on your computer, save yourself a copy. Open it in Photoshop or any similar photo wrangling application. Use the zoom in and zoom out options to replicate viewing the picture close up or from far away.
Option 3. Save the picture and print it out on your laser printer, preferably using fairly high resolution (300dpi is sufficient) and as large as you can on the piece of paper. Stick this to a wall and then view it from far or near.
Option 4. Look at this page, where I've reproduced the exact same image at different scales. This should give you the same effect.
CREDITS This illusion was invented by Philippe G. Schyns and Aude Oliva of the University of Glasgow. It is featured on this web page: http://cvcl.mit.edu/gallery.htm , listed under 'Dr Angry and Mr Smiles'. It is taken from Schyns and Oliva's paper, "Dr Angry and Mr Smiles: when categorization flexibly modifies the perception of faces in rapid visual presentations, Nov 1998". It is a copyright image and if they tell me to take it off this web page, I will.