A faucet, pipes, seven containers… and a seemingly simple question: which one fills first? This type of puzzle is fascinating because it requires less calculation than careful observation. The key here is to resist the urge to follow your instincts and follow the water’s path, as if you yourself were a drop about to fall into the circuit.
The right method: observe before concluding
Identify obstacles: valves, caps, pipes that are too high… many obstacles invisible at first glance.
Read the slope of the circuit: the water obeys the force of gravity and chooses the most direct and least steep path.
Compare lengths: given the same load, the shortest route almost always wins, especially if it has fewer curves and branches.
Keep in mind that until a container is filled to the discharge level (overflow), it acts as a dead end that does not feed the rest.
The classic trap: not all pipes are “open”
The puzzle often hinges on small details: a clogged section of pipe, a drain positioned above the liquid level, an elbow that’s too high. The result: branches that look promising but don’t allow anything to pass through. This is what confuses most people who respond too quickly.
Winning reasoning, step by step
Let’s imagine the scene: the tap opens, the flow descends and splits into two main directions, left and right.
Left: The route to containers 7, 6, 5, and 4 is longer and more winding, with more curves and detours.
Right: The flow is much more direct, leading to 3, 2, and 1 via shorter tubes.
In the right branch, the pipeline then splits into three vertical pipes. The one closest to the branch leads to container 3: it is lower, shorter, and unobstructed. The pipes leading to container 2 and container 1 branch lower or follow a longer route, which delays filling.
Verdict: Container #3 fills up first.
