They make you think, but not too much. When you have a character in your film that is doing something, but we aren't quite sure why, THIS is your track.
Packing a suitcase full of marshmallows? Half Mystery!
Wiring a toaster into the side of a city bus? Half Mystery!
Talking to their mother-in-law about monocot sexuality? Actually... that might be too much for this track.