To solve this water-flow puzzle, follow the pipes carefully and pay attention to which outlet is lowest.
Water enters glass 1 first. Its lower open outlet leads toward glass 3, so the water travels there before it can reach glass 2.
Glass 3 also has two possible routes. The lower outlet on its right side connects directly to glass 7. Because this pipe is open and positioned lower than the route toward glass 6, the water continues into glass 7.
The flow is:
Glass 1 → Glass 3 → Glass 7
Therefore, the first glass to become completely filled is glass 7.
