Note: Larger version that works better right HERE.
Can you see all twelve dots at once? You can't, can you? At first I thought this was an animated gif with dots that blink to screw with me, but it's not, all 12 dots are there all the time, you just can't see them all at once because SORCERY. I mean biology. I don't know what I mean.
It's a variation of the scintillating grid illusion--which itself is a variation on the Hermann grid illusion. Any of these take advantage of lateral inhibition in the retina. In layman's terms, this basically means that the stimulated light receptors in your eye can sometimes mess with the ones adjacent to them, making you see things that aren't there, or not see things that are there.
See, I knew there'd be an explanation that makes no sense to me. I'm convinced I can't see them all at once because my eyes are too far apart. I can't see my nose either, not even if I cross my eyes. You know why? "Your dad got your nose when you were a kid and never gave it back?" I've missed so many smells!
Thanks to Tank, who saw 36 dots at once and may have just had a seizure.