Riddle for the most attentive: can you guess which container will fill up first?

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

Facing the diagram, start with three golden highlights: 

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.

Why this result seems “magical” (when it is logical)

Our brain likes to extrapolate “by eye”: we imagine that the water is distributed everywhere at once. In reality, it favors the   path of least resistance  . Container 3 combines the advantages of direct access, no clogging, low inlet height, and short travel distance. As long as container 3 is not saturated at the outlet (if it has one), it captures most of the initial flow.

Train your eye: a quick checklist to apply to all versions

• Look for dams: a tiny line can mean “blocked.”
• Note the levels: a branch rising above the liquid interrupts its flow.
• Count your elbows: the more you have, the greater the pressure loss.
• Measure the proximity of the first vertical pipe after the main branch.
• Be patient: the puzzle rewards completion, not haste.

A little extra fun

Want to spice things up with family or friends? Hide some sections (print, fold, mask) and ask others to reconstruct the probable route before revealing the full diagram. Guaranteed laughter… and food for thought!

Moral:   In these puzzles, as in life, the shortest and clearest path often wins, provided you pay attention to the details. To proceed quickly, remember  to check the obstacles   before drawing any conclusions.