Camera Obscura

In order to make a camera obscura you need a small dark room with one circular entry of light. This room is now essentially a huge camera. If you place a sheet of paper in front of the light you will see a small flipped images of whatever is on the other side of the entry of light. If you was to place photographic paper into this stream of light you could potentially make a photographic image.

This is a good example of what a camera obscura could look like

As you can see the light entry is on the top and the reflected image is beneath it.