Knock on the door and ask who is serious.

Make sure if there is anyone inside before you enter the door.

When knocking at the door, ask "hello?" Make sure there is no one inside. You can't go in now. Give the other person a hint that you are going in. This is a polite way to enter the door. This kind of expression is common in visitors, couriers or other occasions that need to communicate with people inside the door.

Different knocks on the door mean different things. Generally speaking, knocking at the door means temptation, which means that the knocker is likely to be a stranger; Knocking twice means waiting for the other party to reply, indicating that the knocker knows himself; Knocking three times means asking, which means "hello?" Knocking on the door is a sign of respecting the privacy and space of others.