From concealed creatures to seamless patterns, these illusions test our perception and cognition.
Recent internet sensations include the 'Hidden Horse Among Dogs' puzzle.
A pack of dogs appears to be posing for a photo in this simple image.
A single horse, dressed to blend in with the dogs, is hidden among them.
The challenge: find the hidden horse in six seconds.
On the surface, the image looks to be a group of playful, interested dog breeds.
However, closer investigation reveals a horse cunningly snuggled amid the hounds.