Part of the diagram must be filled with water. The numbers underneath and next to the diagram indicate how many cells of each row and column must be filled with water. Connected areas of filled cells must have the same surface height everywhere - even if the surface not connected, like in a U-shaped tube.
The 4 in the lower right indicates that the whole bottom row must be filled with water. The row above that needs two filled cells. If we would use the two cells in the middle, then the third column would get too much water. Therefore, we have to fill the two outer cells.
The rightmost column needs three filled cells. If we would rise the water level of the bottom two cells, then some would pour into the third column, which is already satisfied, so this is not possible: we need to use the top cell. Now all of the numbers are correct, and we cannot fill any more cells.
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