Use your head: This is the first question in the entrance examination of Harvard University in the United States: Which cup is filled with water first?

From the above analysis, the analysis points of the chart are divided into two aspects:

First, under normal circumstances:

First, tap water will supply water to No.3 after it enters 1 cup (No.3 water supply pipe is lower);

No.2 and No.3 will not supply water to No.4 and No.5 (because the water inlet pipes of No.4 and No.5 are blocked), so No.3 will be full first;

No.3 and No.3 will overflow when they are full, and 1 needs to continuously supply water to No.3;

4. In this way, No.2 will never have water, and No.6 and No.7 will never have water.

To sum up: the 3rd will be full, 1 will be full of water, and the 2nd, 4th, 5th, 6th and 7th will be full.

B. When the water is full and does not overflow:

First, tap water will supply water to No.3 after it enters 1 cup (No.3 water supply pipe is lower);

No.2 and No.3 will not supply water to No.4 and No.5 (because the water inlet pipes of No.4 and No.5 are blocked), so No.3 will be full first;

No.3, No.3 water is full (provided that it will not overflow), and No.2 will be supplied on 1;

No.4 and No.2 will supply No.7 when there is water (because the inlet pipe of No.6 is blocked);

There is water leakage at the bottom of No.5 and No.7, resulting in water dissatisfaction;

No.6 and No.2 need to continuously supply water to No.7, so that No.2 will never be full;

7.No. 1 It is necessary to continuously supply water to No.2, so that No. 1 will not be full.

To sum up: No.3 will be full, 1, 2,7 will have water but not full, 4,5,6 will never have water.