Dutch photographers Anuschka Blommers and Niels Schumm shot a series of photographs for a Dove campaign that uses the Thatcher effect. The effect is created by flipping a portrait upside down while keeping the eyes and/or mouth right side up. The human brain  has a difficult time detecting local feature changes in a upside down face, despite identical changes being obivous in a upright face. Give it a try in Photoshop!!