Take a look at these upside-down faces. Margaret Thatcher and Prince William look pretty normal, right?
Now, click here to see this pair of images right side up!
These are the exact same image, only the first one is upside-down. Why does our brain perceive the upside-down version as normal when it's actually been grotesquely altered?
Scientists are still puzzling this out, but there are a few possible explanations.
One possibility is that our brains rely a lot on expectations, so if we expect the faces to be normal, we will perceive them that way. A related explanation is that we have existing "mental maps" in our brains for common things like faces. Since our "mental maps" always have the facial features oriented one way, it's hard for the brain to override the expected map of the face despite the facial features being flipped around.