Camera obscura
Come inside and visit a camera. The hole in the wall acts as a lens and lets in the light. You can see on the opposite side of the camera how the image is formed, upside down.
The experiment is simply a small hole in the wall of a dark room. The hole acts as a lens and allows light from outside to enter. When the light falls onto a surface, the surroundings of the room appear as an upside down image. Two advantages of a camera obscura over a normal camera with a lens are that the depth of focus is infinite, and the magnitude of the image can be adjusted by changing the distance from the hole. The disadvantage is that the intensity of the light is significantly lower.