Which One Will Fill Up 1st? The Water Tank Logic Puzzle Solved
Connected tank puzzles and fluid dynamics logic brain teasers are huge drivers of viral engagement across social media. They test your spatial reasoning and visual attention to detail by challenging you to trace liquid paths while spotting hidden obstacles like blocked pipes or sealed outlets.
A widely shared visual puzzle shows a faucet pouring water into container 1, which is connected to containers 2, 3, 4, 5, and 6, accompanied by a direct challenge:
“Which one will fill up 1st?”
While the puzzle looks like a straight system flow at first glance, careful inspection of the connecting pipes reveals hidden seals and traps that eliminate most of the options.
In this guide, we break down the path from container to container, analyze every pipe connection, and reveal which tank actually fills up first.
Step-by-Step Flow Analysis
To find the correct container, follow the water flow starting from the tap:
[ WATER FLOW DIAGRAM ]
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[ CONTAINER 1 ]
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-------------------------------------------------
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[ Right Pipe to Tank 2 ] [ Right Opening to Tank 5 ]
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(Open - Flow to 2) (Blocked / Closed Pipe)
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[ CONTAINER 2 ]
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-------------------------------------------------------
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[ Pipe to Tank 4 ] [ Lower Right Pipe to Tank 6 ]
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(High Siphon / Blocked at Tank 3 inlet) (Open Path to Tank 6)
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[ CONTAINER 3/6 ]
1. Examining Container 1
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Water flows directly into Container 1 from the tap.
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Container 1 has two outlets:
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Upper Right Outlet (leading to 5): The pipe connection entering Container 5 is completely blocked at the wall. Water cannot enter 5.
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Lower Right Pipe (leading to 2): This pipe is open. Water flows down into Container 2 before Container 1 can ever fill to the top.
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2. Examining Container 2
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Water accumulates in Container 2.
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Container 2 connects to other containers:
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Left Side Connections (leading to 4 and 3): The pipe going to Container 4 loops upward, but the outlet going down from Container 4 into Container 3 is blocked/sealed at the wall entrance to 3.
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Right Side Connection (leading to 6): The pipe exit on the lower right of Container 2 leads directly into Container 6. The inlet into 6 is fully open.
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3. Examining Container 3 & 4
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The pipe from Container 4 to Container 3 is closed at the entrance to Container 3.
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The pipe from Container 3 to Container 6 is also blocked/sealed at the inlet of Container 3.
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Because the entrances to Container 3 are blocked, water never fills Container 3 or Container 4.
4. Examining Container 6 vs Container 5
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Container 5: The inlet pipe from Container 1 is completely sealed at the entrance. Water never reaches Container 5.
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Container 6: The inlet pipe from Container 2 is fully open and positioned at a lower point. Water flows straight into Container 6.
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Since Container 6 has no open outlets draining out of it, water will accumulate in Container 6 until it is completely filled to the top!
Pipe Status & Tank Verification Matrix
| Container | Inlet Pipe Status | Outlet Status | Will It Fill 1st? | Reason |
| 1 | Open (Faucet) | Drains into Tank 2 | No | Water drains to lower tanks first |
| 2 | Open (From Tank 1) | Drains into Tank 6 | No | Water drains to Tank 6 first |
| 3 | Blocked | Blocked | No | Inlets are sealed shut |
| 4 | Connected | Blocked at 3 | No | Path blocked downstream |
| 5 | Blocked | N/A | No | Inlet from Tank 1 is sealed |
| 6 | Open (From Tank 2) | None (Sealed Base) | YES | Fills completely with no exit drain |
The Answer
Container 3 or 6 is often debated, but looking closely at the pipe connections:
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The path to 3 is blocked at the inlet.
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The pipe to 5 is blocked.
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Container 6 has an open path from 2 and no lower outlets.
Therefore, Container 6 will fill up 1st!
Why 90% of People Pick the Wrong Container
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Overlooking Blocked Pipes: Most viewers trace the general direction of lines without zooming in to see which pipe ends are closed off by solid lines.
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Ignoring Gravity & Elevation: People often assume top tanks fill first, forgetting that liquid naturally drains to the lowest available unblocked reservoir before upper containers can fill.
Conclusion
The correct answer to “Which one will fill up 1st?” is Container 6. By systematically checking for blocked walls and tracing liquid gravity flow, the path directly leads to Tank 6!
Frequently Asked Questions (FAQ)
Which tank fills up first in this puzzle?
Container 6 will fill up first.
Why doesn’t Container 3 fill up?
The inlet pipe connecting into Container 3 is blocked by a wall line, preventing water from entering it.
Why doesn’t Container 5 fill up?
The connecting pipe from Container 1 into Container 5 is sealed shut at the wall.
