Take the shoebox and cut out the side of one end of the box and tape the thin onion skin paper to it. Make a small, round hole in the other end of the box and tape the lid to the box. Make the hole a little larger than a pin hole.
Light travels in straight lines.
This is why the image is always inverted. The light from point A of the object travels in a straight line through the pinhole to point A' on the onion skin of the box, and point B to point B' thus inverting the image. When the pinhole is enlarged, the image will get blurred. The onion skin paper at the end of the box may be replaced with a photographic plate and exposed for a short time. The box has to be held steady.