This is an easy way to produce a home-made automatic door-closer. The bottle (in blue) as shown in the image can apply the force to close the door as it will slide down under gravity. The rope attached to the black pulley above will get tight and pull the door under tension. The door will continue to move as long as the bottle will slide down to full tension. We may open and close the door from both sides. Only, we might forget to shut it after we leave the room. Especially useful for AC rooms. This will cost just as much as a 2 lt Cola bottle and 1 nail. The pulley can be made from the cap of some old container. A little grooving in the side of the cap will be helpful to maintain the rope in line.
Must give it a try. 😉