Some commentators assert that the Torah is composed of black fire on white fire (i.e., the black letters of the Torah and the white parchment on which the letters are written, respectively).  The three and four-pronged shins on the tefillin Shel Rosh reflect this: The three pronged shin is a black fire shin.  The four pronged shin can be seen as a three pronged-shin in white fire (i.e., a three pronged white fire shin is created by the white space surrounding the four pronged shin).  In this sense, maybe the tefillin reflect the Torah, or God, as a whole since they have complementary black and white fire elements.

The kids in the 6th Grade classes were, somewhat to my surprise, very interested in this line of thought.  It led to related discussions of the role of uncertainty in Judaism, the white fire/invisible Bet in the middle of the letter Peh, how there must therefore be a white fire Peh surrounding the letter Bet with which the Torah begins, how this white fire Peh (which is also the Hebrew word for "mouth") could represent God speaking the world into existance, and how an implied white fire Aleph to the right of the Peh at the begining of the Torah might symbolize how God created the world from nothing (since the Aleph is a letter with no sound)!    

It was fun to learn about this as part of the BAP program to to share this with the students.  Sixth graders are more sophisticated that I had previously thought!