In physics experiment, make a hole in the bottle, fill it with water, unscrew the bottle cap, and water will flow out of the hole. I know the experimental principle of this experiment. why

Fill it with water and tighten the bottle cap. Because the inside of the bottle is isolated from the outside of the bottle, no water flows out of the hole in the side wall under the action of external atmospheric pressure. After the bottle cap is loosened, the atmospheric pressure inside and outside the bottle is the same. Because the liquid is affected by gravity, the pressure of the liquid increases with the depth, and water flows out of the hole in the side wall. This experiment proves the existence of atmospheric pressure.

Extended data:

Gas is composed of a large number of irregularly moving molecules, and these molecules are bound to collide with objects immersed in the air constantly. Every collision, air molecules will give an impact force to the surface of the object, and the result of continuous collision of a large number of air molecules is reflected in the atmospheric pressure on the surface of the object, thus forming atmospheric pressure.

The more molecules per unit volume, the more times air molecules collide with the surface per unit area in the same time, and the greater the pressure.