The key to this puzzle is to work out the positions, sizes and timing of the rain drops that created the ripples. - There are four sets of concentric ripple circles, whose centres give us the four rain drop positions. - The thickness of the ripples determine their drop sizes. There are two visible types, with the thicker ones being roughly three times as thick as the thinner ones. - The timing of the ripples can be deduced from the ripple sizes. The larger the ripple circles are, the earlier they dropped. As hinted by the title, the rain drops should be interpreted as Morse codes, with the smaller drops (the thinner ripples) corresponding to dots and the larger drops (the thicker ripples) corresponding to dashes. The four drop positions give us four sequences of dots and dashes, ordered by the timing of each drop. The following letters are obtained: _ _ _ O . _ . . L . _ _ W . . _ . F There are a few possible anagrams of these four letters, but if we order them by the timing of the first drop (the largest ripple size), we get FLOW as the answer, consistent with the theme of the puzzle. |

The answer is: flow |
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