The tube guitar (or as we at Bash the Trash call it - the Tubetar) is a step up from rubber band instruments.
MUSICAL INSTRUMENT BUILDING - THE PROCEDURE!
You can pluck the strings with your fingers or strum it with some kind of pick sort of thing (a penny is acceptable, although stiff plastic is better). You can also change the tension of the strings (tighter = higher and looser = lower) by playing like a brazilian berimbao or a japanese koto. Play the longer set of strings while pushing on the strings on the other side of the bridge. The more you push, the higher the pitch.