Whether or not your child watches much television, there's bound to be a time when they ask how it works. For all they know, there are people living in a little box, putting on each show. Explaining how things work is part of parenthood, and in the case of electronics, it can be a good opportunity for your child to learn about some basic elements of science. A television is a complex appliance that incorporates many scientific areas: magnetics, electrons, electricity, etc. Using general terms will help.
Explain that some people refer to the television as "the tube," because in many older televisions, there is a tubelike mechanism (a cathode ray tube). It sends a ray of focused, heated electrons (or just say a ray of light) through a vacuum device that bounces it directly onto the back or inside of our television screen. Newer televisions have liquid crystal displays (LCDs) that use less energy. These televisions have liquid crystal placed between two panels that react to high temperatures and electricity. The crystals either allow light through or block it, and the pattern forms the image when it's lit from behind.
Explain that in both cases, the television screen glows when a ray of light hits it. When it glows, it comes through to our line of vision in a bunch of red, green and blue dots.
Explain to your child that television screens, like computer screens, take a bunch of tiny dots and put them together to make an image. Our brains are able to process the dots and create the image. Let your child look closely at the screen to see if he sees the pixels.
Explain that these dots all form a picture when the screen is "painted." That means that a beam literally paints or scans the television screen at a pace of 60 times per second. The dots and fast screen painting make it appear that the images are in motion. It's not much different than the flip books that kids enjoy.