Water Tank Puzzle #4: Which Tank Will Fill Up First? Correct Solution

Water tank logic puzzles are extremely popular on social media platforms like Facebook. They challenge viewers to combine visual attention to detail with fluid dynamics logic, testing who can spot hidden pipe blockages and open ends.

This well-known puzzle, labeled Water Tank Puzzle #4, features a faucet filling Tank 1 and asks:

“Which tank will fill up first?”

While many participants prematurely guess Tank 3, Tank 4, or Tank 10, a thorough inspection of every single connection reveals hidden traps, open bottoms, and blocked pipes.

In this guide, we break down the exact path of the water step-by-step to reveal which tank will actually fill to the top first.

Step-by-Step Path Breakdown

Let’s trace the flow of water starting from the tap at the top right:

                         [ WATER FLOW PATH ANALYSIS ]
                                      |
      -----------------------------------------------------------------
     |                                |                                |
 [ FAUCET & TANK 1 ]         [ TANK 3 TO TANK 4 ]            [ TANK 4 TO TANK 5 ]
 • Water enters Tank 1       • Tank 3 bottom is BLOCKED      • Long left pipe connects 
 • Pipe to Tank 2 BLOCKED    • Water fills 3 to outlet         Tank 4 directly to Tank 5
 • Water flows to Tank 3       and flows to Tank 4

1. From Tank 1 to Tank 3

• Water drips into Tank 1.

• The pipe leading from Tank 1 to Tank 2 is blocked at the entrance to Tank 2.

• Water is forced to flow into Tank 3.

2. From Tank 3 to Tank 4

• The lower exit of Tank 3 (leading right toward Tank 8) is blocked.

• As water rises in Tank 3, it reaches the upper left pipe and flows into Tank 4.

3. From Tank 4 to Tank 5

• Water leaves Tank 4 through the left side pipe.

• The pipe runs all the way down the left side into Tank 5. (The bottom branch of this pipe leading toward Tank 13 is blocked at the junction near Tank 13).

4. From Tank 5 to Tank 6 & 7

• From Tank 5, water flows through the right outlet into Tank 6.

• From Tank 6, water flows through the right outlet into Tank 7.

5. From Tank 7 to the Lower Network (Tanks 8, 9, 10, 11, 12, 13)

• Looking at Tank 7: The upper outlet into Tank 8 is blocked.

• Water flows down from Tank 7 into Tank 9.

• Tank 9 has an open bottom (two open pipes extending downward into nothing), so water would drain out—except the path connects directly down toward Tank 10.

• Looking at Tank 10: The entrance at the top right from the outer loop is blocked, but the line from Tank 9 feeds into Tank 10. However, Tank 10 has an open bottom pipe connecting to the lower horizontal channel.

• Looking at Tank 11, 12, & 13:

  • Tank 11 has a blocked entrance from Tank 7.

  • Tank 12 has a blocked pipe entering from Tank 6.

  • Tank 13 has a blocked pipe at its bottom connection.

• Tracing the complete open pathway shows Tank 3 cannot hold water permanently because it drains into Tank 4, which drains to Tank 5, Tank 6, and Tank 7. Since Tank 7 drains through Tank 9 into Tank 10, and Tank 10 is the first fully sealed container at the end of the open line, Tank 10 (or Tank 3 in simplified versions where lower blockages isolate it) is the target. In this exact diagram, Tank 3 fills up first once its outlet to Tank 4 is reached and Tank 4 fills, but strictly following fluid level: Tank 3 is the first container to reach capacity!

Summary of Key Pipe Connections

Conclusion

By carefully eliminating blocked entrances at Tanks 2, 8, 11, 12, and 13, the continuous path carries water through Tanks 1 $\rightarrow$ 3 $\rightarrow$ 4 $\rightarrow$ 5 $\rightarrow$ 6 $\rightarrow$ 7 $\rightarrow$ 9 $\rightarrow$ 10. Depending on whether you measure the first container to reach maximum level or the final sealed reservoir, Tank 3 (or Tank 10) is the correct answer!

Frequently Asked Questions (FAQ)

Why doesn’t Tank 2 fill up?

The entrance pipe connecting Tank 1 to Tank 2 is completely closed off by a solid wall barrier, blocking any water from entering Tank 2.

What is the trick to solving water tank puzzles?

The key trick is to ignore the overall complexity at first and zoom in on every single pipe junction. Check both the entrance and exit of each pipe for solid lines (blockages) or missing bottoms (drains).