3. Here is the idea is to write the shortest possible piece of music. So you give up all the advantages that length gives you (repetition, development, contrasting sections, etc..). You can conceive it either as a time-limit exercise (five seconds) or as a way of getting as short as possible, to the limits of human hearing.
A sequence of two or three chords, but each very complex and multilayered, would be one approach. You be basically substituting the vertical dimension for the horizontal one. Another would be to take a piece and mechanically speed it up so that it fits the time frame. A lot can happen in 5 seconds at 600 bpm.