In fact, siphoning is not entirely caused by atmospheric pressure, and siphoning can also occur in a vacuum. The force that lifts the liquid upward is the cohesive force between the molecules of the liquid. When the siphon phenomenon occurs, the liquid will continue to flow in one direction because the liquid flowing out of the tube is more than the liquid flowing into the tube, and the gravity on both sides is unbalanced.
   As the liquid flows into the tube, the higher the pressure, the lower the pressure. If the tube where the liquid rises is very high, the pressure will drop to the point where bubbles (composed of air or other gaseous components) are created in the tube, and the height of the action of the shower drainage siphon is determined by the generation of bubbles. Because the bubble will break the liquid, the force between the gas molecules at both ends of the bubble is reduced to 0, thus destroying the siphon effect, so the tube must be filled with water.
   Under normal atmospheric pressure, the siphon works better than in a vacuum, because the atmospheric pressure on the nozzles on both sides increases the pressure inside the entire siphon. Application example: Insert one end of the hose filled with water into the water, and the other end hangs out of the water container, and the water outlet should be lower than the water surface. This way, the water will flow out of the container down the hose.