Every human has a blind spot in each of their eyes. Position your face between the
X and the
I below focusing on the
dot between them. Close your right eye and look across your face at the letter
I with your left eye. Now move your face back and forth at the screen, at a certain distance the
X should vanish somewhere between 30cm away.
_______________X_________________.______ ____________I
This happens to all animals except squid, so if it doesn't work for you then the only logical conclusion is that you are a squid.