Why is there a small hole in the insert of the electric plug?

There is a small hole in the front of the two inserts of the plug, but sometimes it can be seen that the plug has no small hole. Why is there a small hole in the plug? There are small protrusions on both sides of the socket. When the plug is inserted, the small bumps on the socket just snap into the small holes on the insert from both sides, which can prevent the plug from falling off. Why do you hear a "buzzing" sound when you put your ear close to the bottle mouth and put the shell on your ear? You can also hear the sound of "buzzing", like the waves of the sea. You can also hear this sound by gently covering your ears with your hands. This is because there are always all kinds of sounds around us. There is almost no sound environment on the earth where we live. These sounds produce * * * sounds with the air in the bottle, and the "buzzing" sound can be heard when the ear is close to the bottle mouth. This is one reason. Another reason is that the bottle mouth is very close to the ear.