Why do some caps need to be pressed down before they can be unscrewed?
That's because the connection must be pressed down before it can be used to force it out. Mainly to prevent the use of the process, because unscrewed, there will be residual surface, or direct contact with the body and so on.
The air in the bottle is air pressure. Under the pressure, the air in the bottle can be sunk, and the bottle can not be crowded in the bottle mouth, and the pressure can be reduced to a certain extent, so that the lid can be opened conveniently.
