This is an entry for the 16th Fortnightly Challenge.
What is this famous puzzle?
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This is the 9-weights puzzle.
Or more specifically,
You have 9 balls, equally big, equally heavy - except for one, which is a little heavier.How would you identify the heavier ball if you could use a pair of balance scales only twice?
The solution is, as depicted,
to take them in groups of three, and weigh two of them, so that you know which group contains the target weight. Now pick any two in that group and compare them, to finally find the target weight.