[Question] What are fun little puzzles /​ games /​ exercises to learn interesting concepts?

Things in the category of the 2-4-6 puzzle where you need to guess the rule and can try as many examples as you want to figure out the puzzle, and it teaches ‘gung jr ner cebar gb pbasvezngvba ovnf’ (https://​​rot13.com/​​).

Please describe the puzzle /​ game /​ exercise and its solution /​ strategy /​ take-away separately in order to avoid spoiling it.

Am particularly interested in cognitive biases, but don’t want to restrict the answer-space to those.

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