A. It is called the “Luz” bone. According to the Medrash (Kohelet Rabba 12:5) this bone will be used for the resurrection of the dead. One source has the Luz at the base of the spine, and the other has it atop the spine, where the back knot of Tefillin rests. It is said that this bone is especially nourished from food eaten at the Melava Malka (farewell to the Sabbath Queen) meal after the end of the Sabbath.