The best way to solve this type of illusion is to examine the picture slowly instead of trying to count everything at once. Begin in one corner and move across the image, looking carefully at the negative spaces between the dogs as well as the markings on their bodies.

Pay special attention to areas where two shapes meet. A dog’s ear may double as part of a human face, while a patch of shadow can form an eye or mouth. Turning the image slightly or stepping farther away can also make some of the hidden faces easier to recognize.

This is why people often report different totals on their first attempt. Once your brain locks onto the animal shapes, it becomes surprisingly difficult to switch interpretations. After spotting one face, however, the rest usually become easier to notice.

So before checking the solution, take one more careful look. Count every complete human face you can identify and make sure you have not counted the same one twice. The answer depends on the exact puzzle image, so the final number should only be revealed after verifying the original picture itself.