A door opens slowly. At first, the visitor is still partially hidden. Only at the end do you see him completely.
Exercise: Use the drawings of the opening door to come up with something else that wants to enter the room. It could be a human, an animal, a alien, a river, a pile of balls, whatever. If you find a door too difficult, you can also make a round hole or a square hole in the wall that will reveal something.
A ship comes in through a hole in the wall!