Meaning: a piece of wood, glass, or metal, which is moved to open and close the entrance to a building or room.
Language development (shown in the video): Age 1;8, the baby Juli signed the ASL word "DOOR" and pointed to it (or was she asking whether it was a door?). Age 1;1, Juli commented that "the door is stuck". Age 2;4, she requested her grandmother to close the door (and confirmed, "yes"). Age 4, Juli posed a hypothetical question: "If bad people come, what would you do? (Rhetorical question with raised eyebrows.) Lock the door. (With her tone of emphasis.)"