21grams = your weight of your soul!
Most of those of a religious bent believe in life everlasting for the faithful, a continuation of the life force that reaches far beyond the limitations of mortal flesh. In such belief systems, death is not an end but a transformation: though people shed their corporeal selves at the moment of demise, that which made them unique beings lives on to rejoin the Creator. We call this intrinsic personness "the soul," an entity described in the dictionary as "The immaterial essence, animating principle, or actuating cause of an individual life."
Yet as much as we believe in the concept of "soul," this life spark remains strictly an article of faith. As central as it is to our perception of ourselves, it can't be seen or heard or smelled or touched or tasted, a state of affairs that leaves some of us uneasy. Without the soul, dead is dead. But if it could be proved to exist, a great deal of anxiety over what happens to us when we die would be vanquished.
Enter Dr. Duncan MacDougall of Haverhill, Massachusetts. The doctor postulated the soul was material and therefore had mass, ergo a measurable drop in the weight of the deceased would be noted at the moment this essence parted ways with the physical remains. The belief that human beings are possessed of souls which depart their bodies after death and that these souls have detectable physical presences were around well before the 20th century, but claims that souls have measurable mass which falls within a specific range of weights can be traced to experiments conducted by Dr. MacDougall in 1907.
MacDougall seems not to have made any more experimental breakthroughs regarding the measurement of the human soul after 1911 (at least, none considered remarkable enough to have been reported in the pages of the New York Times), and he passed away in 1920. Nonetheless, his legacy lives on in the oft-expressed maxim that the human soul weighs 21 grams. (At the moment of death, MacDougall's first test subject decreased in weight by three-fourths of an ounce, which is 21.3 grams.)