The eyes of some pictures seem to follow you around the room, like those of the famous WWI recruitment poster which helped garner almost 3 million volunteers in two years:
Try it. Get up and look at your screen from the side. Is he still looking at you? He should be.
Recently published research in the journal Perception  discusses how this effect works. The story was covered in the press (e.g. here). Turned around into a ‘how to’ rather than a simple ‘explanation’ it’s perfect material for a hack. I saw it too late to include in the book so I’m putting it here.
So here we go: How can you design pictures of faces with eyes that will follow you round the room?