In 1911 Franz Kafka wrote in his journal: the hidden in a "Vexierbild" is obvious and invisible. Obvious to the one who found what he was invited to look for, invisible to the one that is unaware to look for something. It is similar with good ideas; once they are there they seem obvious. To start a creative workshop by looking at pictures with hidden faces and trying to unravel them is fun and stimulates the participants to look and think carefully. "I spy with my little eye" creates a relaxed atmosphere and fosters self-confidence.
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https://de.wikipedia.org/wiki/Vexierbild
https://en.wikipedia.org/wiki/Hidden_faces