Live Body, Lighter Body; Dead Body, Heavier Body

Did you ever wonder why a dead body is heavier than a live body?
When R. Yochanan Ben Zakai uncovered his plan to sneak out of the besieged walls of the city of Yerushalayim to visit the General Vespasian by faking as dead in a coffin, he instructed his students that only they shall carry his coffin. The Gemara (גיטין נו, א) explains that he knew that a dead body weighs more than a live one, and thus, he did not want anyone to uncover his disguise by detecting the unusually light coffin.
But why is this indeed so - what makes the body lighter while alive?
R. Bechaye explains that the living person is comprised of a material body and a spiritual soul. The soul enlivens the body and "carries" it, thus making it lighter. When the soul departs upon the person's death, the body is left in its heavy material form with nothing carrying it. This is the true meaning of what the Gemara (שבת צד, א) says "החי נושא את עצמו - a living body carries itself."
The same can be found with inanimate objects as well: When one takes molten iron directly from the fire, it is lighter than when after it cools off. This is because immediately after emerging from the fire, the metal still has some of the fire - the soul - in it; this "soul" carries the metal, thus making it lighter.
רבינו בחיי עה"ת שמות לב, טז
ועי' "המעין" שנת תשד"מ גליון תשרי ע' 55 מאמרו של חיים סבתו בענין חי נושא את עצמו, ותשובתו בצידו ב"המעין" שנת תשד"מ גליון ניסן ע' 63
See here