Spring fitted doors close by themselves when released. You want to keep the door open for a long time, say for an hour. If you put a half kg stone in front of the open door, it does not help. The sotne slides with the door and the door gets closed. However, if you sandwitch a 20 g of wood in the small gap between the door and the floor, the door stays open. Explain why a much lighter peice of wood is able to keep the door open while the heavy fails.