According to Wikipedia, "The Thatcher effect or Thatcher illusion is a phenomenon where it becomes more difficult to detect local feature changes in an upside-down face, despite identical changes being obvious in an upright face".
In other words, your brain is too stupid to easily recognize changes to the eyes, mouth, nose, etc. when viewing an upside-down face.
Below we start with an image of 
Now we will simply flip him upside-down.
Does it look funny to you?
I hope not because it shouldn't.
Okay now is where the fun begins.
Using Photoshop I have inverted
That is until we vertically flip the edited image to reveal what looks like a monster.
@danthemanThanks Dan!
