The Roman emperor Hadrian (may his bones be ground, the rabbis add in a growl) was fond of cornering Jewish sages with theological questions. One day he turned to Rabbi Joshua ben Chanania and asked the classic one.
From what, he said, will the human frame be reconstructed at the resurrection of the dead? If the body decays entirely, what is there left to rebuild?
Rabbi Joshua had an answer ready. From the Luz, he said. A small bone in the backbone, the size of an almond. It is the seed bone. Every bone of the body will grow from it when the time of resurrection comes.
Prove it, said Hadrian.
The rabbi was happy to. He took a Luz bone and dropped it into water. It did not soften. He put it in fire. It did not burn. He put it in a mill and set the stones turning. It could not be pounded. He laid it on an anvil and struck it with a hammer. The anvil split and the hammer shattered, but the Luz bone sat there whole.
That, said Rabbi Joshua, is how God can rebuild a human being. He hides an indestructible seed in the spine, and at the right moment He waters it.
This teaching from Midrash Kohelet Rabbah 12:5, referenced also in the Zohar (Genesis 206), preserved in Hebraic Literature (1901), is the rabbinic answer to the riddle of bodily resurrection. Somewhere in you, they insisted, is a piece that even fire cannot touch.