I put a piece of tracing paper over the back of a camera and showed a group of kids and parents the upside down image projected on the film plane. They were pretty amazed at that. You can't make a digital camera at home, but you can make an analog camera at home. From camera obscura to the latent image on the film emulsion to developing and enlarging. It's all very fascinating. Pinhole day is coming up next month, you could bring in a pinhole camera and demonstrate the paper negative. You could also explain the difference between 8x10 film vs. a digital 35mm sensor (without going into too much detail).