This one is my favorite- a shark jaw forms a doorway to a sort of cavern, within which a dancer-like form cycles between two poses, while the entrance to the cavern fades in a cycle between transparent/opaque/transparent/ (using polarizers, perhaps).  The two cycles go in and out of phase with one another, while some lone teeth mounted on magnets spin erratically just outside the entrance.

(click to view full size) IMG_4215