First, click any panel below to bring up the stereo view. This will look like two identical images side by side.
Lean back and cross your eyes so that they see four total images instead of the normal two.
Then they slowly uncross your eyes until two out of the four images overlap each-other.
Once the middle two images are perfectly aligned you start to see a single 3D image in the center.
Other people get close to the screen and let their eyes loose their focus and their brain just does the rest.
NOTE: Want to make a stereoscopic comic like this? I wrote a blog post explaining how.