The content of a medium is always another medium. This is related to the fact that media "amplify or accelerate existing processes": the content of a medium is the older medium that is amplified or accelerated by the new medium.
"The medium is the message" refers to the fact that, despite common wisdom, it is not the content of a medium that is its message, but rather the form of the medium itself. The content tends to be a distraction that blinds us to the form and effects of the medium.
|Electric light||No content|
|Speech||A nonverbal process of thought|
|Movies||Novel, Play, or Opera|