To make the Where's Waldo pictures challenging, the creator, Martin Hanford, must make sure that all the people in the picture are closer together, thus making it more difficult to pick out Waldo from the background. In this way, Hanford is playing with the grouping principle of

Question

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The principle of proximity suggests that objects that are close to each other tend to be perceived as a group. By placing people closer together, it becomes more challenging to distinguish Waldo from the crowd.

To Make the Where's Waldo Pictures Challenging, the Creator, Martin