Diagram of a camera obscura, 1910.
image by Alex Spade: commons.wikimedia.org
The camera obscura is a camera in its most basic form. Latin for "dark room," a camera obscura can be made from just about any lightproof container by simply poking a hole in it. A small hole can focus light reflected from objects in front of the camera as it passes through it, sharpening the image and projecting it onto the back wall of the camera. The camera obscura can be used as a drawing tool, as a means of taking permanent photographs or, like the one described here, a simple tool for learning about the principles of photography and light.
Paint the inside of the oatmeal container and its lid black, paying extra attention to the lid, especially if it is a light color or translucent---any light that gets into the camera will fade or ruin the image. Set the container and lid aside to dry.
Draw a straight line around the oatmeal can's circumference, about 5 inches from the bottom.
Cut along the line with the craft knife to remove a portion of the can's lower half.
Turn the smaller, bottom portion upside-down. Puncture a hole in the center of the can's bottom using the nail and hammer. This will be the camera obscura's "lens."
Cover the open end of the canister with a sheet of white tissue paper, keeping it taut by pulling the excess at the edges down against the sides of the canister. Secure the tissue paper in this position using duct tape. The camera obscura will project its images on this makeshift screen, which you can then view from the opposite end of the canister.
Cut a small hole in the center of the lid. You'll be viewing images on the tissue screen through this hole, so make it big enough to allow you to look through it, but not so big that light will be able to leak in and ruin the image.
Line up the open end of the larger, lidded half of the canister with the tissue-covered end of the other. Tape the two pieces together, taking care to cover all cracks to prevent light leaks.
Point the bottom of the canister at a well-lit object or scene---preferably outdoors---and look through the hole in the lid. You should be able to see the image on the tissue paper, upside-down and backwards. The image is inverted because light moves linearly---light reflected from the right side of an object enters the lens at an angle and lands on the left side of the image plane, and vice-versa.