Knock before you enter.

Somebody knocked at the door while we were having supper.

However, I was so convinced it was a great book that I started knocking on doors.

What was his surprise at seeing him go up to a door and knock, and to hear him say, "Dear little sister, let me in.

She went in without knocking, and hurried upstairs, in great fear lest she should meet the real Mary Ann, and be turned out of the house before she had found the fan and gloves.