Tefillin are a set of square leather boxes which contain various handwritten scriptures and are worn by observant Jews throughout the world.  Specifically, the boxes are positioned such that one rests on the head, while the other rests on the heart and are held in place by leather straps.  Tefillin are worn every day except for the Sabbath and most Jewish holidays.  Boys and, in some synagogues, girls begin wrapping Tefillin in preparation for their Bar or Bat Mitzvah, and continue the practice throughout their lives.