Update: Jury 1st Prize winner at the 2011 International Puzzle Design competition
Two open-ended Superstrings with the correct spin can be merged into a super-symmetrical cube.
The idea for this puzzle emerged when I was designing shapes based on two interlocked tetrahedra. Inspired by ideas from 'string theory' and 'super-symmetry', I began to search for shapes that were both symmetrical and as similar as possible.
Taking the two pieces apart is not too difficult; reassembling them back into a cube can be more challenging. Hold one of the pieces still; move the other piece through the correct sequence of lateral and rotational moves.