La espiral

para que puedas ver ésta espiral moverse, mira fijamente la cruz del centro, aleja y acerca tu cabeza de la pantalla y veras la espiral moverse!

For this optical illusion to work,
stare at the centre crosshairs
move your head in and out from the screen.
Sometimes when you just look to the side or relax your eyes a bit,
you'll notice the spiral moving