This simple pinhole camera demonstrates that light goes in a straight line, which is one of the basic principles of how any camera works. Here you can make your own simple camera. You can adjust the distance between the screen and the pinhole by sliding the inner tube in and out of the outer box and discover how the image size changes.
Great tutorial on taking an old room in your house and turning it into a dark room. Includes pictures and a complete list of materials needed to develop your own film.