Several warehouses store chemical components X, Y and Z, which when combined can blow up the entire building. Safety regulations prescribe therefore that they are not allowed to be stored close to each other. Also, not too much of the same component can be stored close to itself, so as to avoid nuclear fission or spontaneous combustion.
Enter an X, Y, or Z into every cell of the diagram. Three cells that touch each other cannot all contain the same element, or all different elements.
Consider the top cell. Suppose it contains a Z. Then the cell below it must also contain a Z, but there are only three Z's in the puzzle, so that can't be the case. Now suppose the top cell contains a Y. Then the cell below it cannot be anything, so this is also not possible. So, the top cell contains an X.
Now, the rest of the puzzle follows easily. The cell below the top one must be a Y, the cell to the lower left of that must be a Y too, the cell to the lower left of that must be an X, the cell to the right must be a Z, and the final cell must also be an X.
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