Do you know the siphon phenomenon? The water in the container can automatically flow out through the bend above the surface of the container, just like something "sucks" the water out of the container.
Why does water automatically flow through high bends? It turns out that this is because the pressure difference is at work. When the pressure of the same liquid level on both sides of the elbow is different, the water (or other liquids) in the pipe will flow towards the side with lower pressure. The diagram on the left shows how this pressure difference makes water flow.
To produce a siphon phenomenon, the water flow outlet of the elbow must be lower than the water inlet. Assume that the horizontal plane at the inlet end is A and the horizontal plane at the outlet end is B, and the atmospheric pressure acting on the two horizontal planes upward is P0, but there is a section of the water column with a length of h below the surface A in the right pipe, so the pressure on the same surface with A in the right pipe P=P0- ρ Gh, that is, Po ＞ P, will cause the water column in the pipe to flow to the right pipe, and finally flow out from the B end. From this, we can define the siphon phenomenon: the phenomenon that liquid flows by itself due to the pressure difference caused by the height difference of liquid levels at both ends of the connector is called the siphon phenomenon.
After careful analysis of the schematic diagram of the siphon, it can be seen that the elbow should be filled with water in advance, and the whole elbow should not be filled with air, otherwise, the pressure of the internal air will change the relationship between the pressure difference at both ends, and siphon cannot occur.