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Orientation can have a powerful influence on recognition because people are accustomed to seeing familiar objects from certain viewpoints. Faces provide a particularly clear example: people generally recognize normally oriented faces more readily than unusual or inverted versions. Hidden-image puzzles can exploit similar perceptual tendencies by placing recognizable features in unexpected orientations. When the picture is rotated, the relationship between those features changes from the viewer’s perspective. The new arrangement may correspond more closely with a familiar shape stored in memory. Suddenly, an animal that seemed invisible becomes surprisingly obvious.
That does not mean everyone will experience the illusion in exactly the same way. One person may identify the donkey almost immediately, while another might examine the image for considerably longer before seeing it. A third person may need the rotation clue before the hidden figure becomes recognizable. These differences are normal in recreational visual puzzles. Attention, familiarity with similar illusions, expectations, viewing conditions, image size, and many other factors can influence what someone notices first. The puzzle is therefore better treated as entertainment than as a competition.