A pinhole camera is an interesting way to learn about the refraction of light and about images. To make a pinhole camera, you must pierce a small hole into a sheet of lightproof material that has a dark box behind it. Light sensitive film or paper is inserted into the back of the box opposite the hole, and the hole is pointed at an object or scene for a length of time. Light from the object enters the hole, is refracted, and creates an image on the film.
Refraction occurs when light waves are slowed down. This can happen when light goes from one medium to another, for instance from air to water, or it can occur when the light passes through a narrow opening. In the pinhole camera, light waves get "caught" and slowed down by the edges of the hole, and their path gets slightly bent toward the center of the hole. The paths of the light beams cross inside the box, so that when they reach the film, light from the top of the hole is now forms the bottom of the image, and vice versa. This creates an upside-down image.
Here is a link with a diagram that shows how a pinhole camera works, along with the math for sizing the box and other helpful information.