Could you write an instantly recognisable and world-famous theme using just two notes? That's what John Williams managed with the menancing theme from Jaws.
To recreate the effect yourself just pick a note low down on the piano, and follow it by a note a semitone (one half-step) higher. Make sure you play the first note long and the second note staccato (very short). Something like this:
Then all you need to do is repeat the two notes over and over, starting off slowly and getting faster and faster. Hey presto - instant shark music!