This is your brain making things up.
What you see isn't really there.
Even if I tell you "this isn't what you think," you'll think it anyway — until I make a simple move, and suddenly — you know.
These illusions were created by an artist who calls himself Brusspup. What he does is an exercise in anamorphosis, a conjuring trick that takes advantage of how our brains make sense of the world. If you know how, you can create an image which makes no sense until the viewer happens onto a particular — and it's a very particular — spot. Once the viewer finds the right angle — the only place where he or she can see what the artist intended — suddenly, boom!- the drawing leaps into three dimensions.
That's because in that precise location, the brain is presented with an impossible, contradictory set of inputs that it automatically reassembles into a coherent illusion. You know it's not true — all you have to do is step an inch off your position — but when it's there, it seems uncannily real.
I once wrote a post called Woman On Street Attacked by Giant Snail, It Seems that breaks this down visually. I used the chalk drawings of a street artist to show how it's done. Check that out, if you like. The cool thing about Brusspup, is he not only creates wonderfully cool illusions, he then posts the keys to what he's done on the You Tube site, so you can do them yourself.