Principle: press the outside of the bottle by hand. When the air inside is compressed, the pressure will be transferred to the water, and the water will be pressed into the bottle, compressing the air inside the bottle. At this time, some water will enter the magic bottle, and the gravity will increase, which is greater than the buoyancy, and it will sink. When the hand leaves the bottle, the air volume on the water surface outside the magic bottle increases and the pressure decreases, and the compressed air in the magic bottle pushes the water out. At this time, the gravity of the magic bottle is less than its buoyancy, so it floats upward. When the hand exerts proper pressure on the outside of the bottle, the magic bottle is suspended at any depth in the water.

The pressure per unit area of an object is called pressure, and the symbol is P. Pressure is used to compare the effect of pressure. The greater the pressure, the more obvious the effect of pressure. The calculation formula of pressure is: pressure = pressure/stressed area, that is, p=F/S, the unit of pressure is Pascal, abbreviated as Pa, and the unit is Pa. The methods of increasing pressure are: increasing pressure under the condition of constant stress area, reducing stress area under the condition of constant pressure, or reducing stress area while increasing pressure. The methods of reducing pressure are: reducing pressure under the condition of constant stress area, increasing stress area under the condition of constant pressure, or increasing stress area while reducing pressure.

The liquid exerts pressure on the side wall and bottom of the container, and the pressure increases with the increase of liquid depth.