It's the weight of five nickels, a chocolate bar, a hummingbird - and, according to the acclaimed movie of the same name, the amount of weight we lose at the precise moment we die, perhaps as the soul escapes the body.
Chalk the idea up to urban legend, because doctors say there's no modern physiological evidence of the phenomenon. But there is, it turns out, a historical basis for the claim - albeit one based on scientifically flawed experiments.
Tucked away in a 1907 journal called American Medicine is the story of Dr. Duncan MacDougall of Haverhill, Mass., who postulated that the soul had substance and, therefore, a measurable mass.
To test his hypothesis, MacDougall did something that would be unthinkable today: He placed six terminal patients on a specially designed bed built on a scale and weighed them as they lived their last hours. His first subject was dying of tuberculosis.
"The patient's comfort was looked after in every way, although he was practically moribund when placed upon the bed," MacDougall wrote. "He lost weight slowly at the rate of one ounce per hour due to evaporation of moisture in respiration and evaporation of sweat.
" ... At the end of the three hours and forty minutes, he expired and suddenly coincident with death the beam end [of the scale] dropped with an audible stroke, hitting against the lower limiting bar and remaining there with no rebound. The loss was ascertained to be three-fourths of an ounce."
Or 21.262142347500003 grams.
MacDougall claimed that the weight loss couldn't have come from moisture evaporating because he had accounted for that. The patient hadn't moved his bowels, he said, and his bladder "evacuated one or two drams of urine," which remained on the bed.
Also, MacDougall asserted, the drop wasn't from the loss of left-over air in the lungs, because the doctor and a colleague each got on the bed, strenuously inhaled and exhaled, and saw no change in the scale.
"Is it the soul substance? How other shall we explain it?" he concluded.
Three other expiring patients MacDougall studied over the course of several years also lost weight - either a half-ounce or three-eighths of an ounce - which he said couldn't be explained.
MacDougall discarded results from two more patients, one because of problems with the scale and outside interference, the other because the patient died within five minutes of being placed on the bed and MacDougall wasn't ready.
"I am aware that a large number of experiments would require to be made before the matter can be proved beyond any possibility of error," MacDougall wrote, "but if further and sufficient experimentation proves that there is a loss of substance occurring at death and not accounted for by known channels of loss, the establishment of such a truth cannot fail to be of the utmost importance."
Though it's unclear whether anyone tried to reproduce his work, it is unlikely. MacDougall tried the experiment with 15 dogs - he had to drug them to keep them from struggling - and recorded no loss of weight at death.
Canines aside, Ronn Wade has been pondering such deep questions all his life - or at least since the age of 7, when he began helping his father, a mortician, embalm dead bodies.
As director of the Maryland Anatomy Board, Wade oversees the state's body donor program. As head of the anatomical services division at the University of Maryland School of Medicine, he prepares bodies used by doctors and doctors-in-training in the name of science.
According to the laws of physics, he points out, energy is never lost, it just changes forms. So if the spirit is a kind of energy, what happens to it?
"It's one of those metaphysical questions that hard science doesn't have an answer to," said Wade, who hasn't seen 21 Grams, in part because he prefers reading to film.
He wonders how MacDougall determined the exact moment of death. After all, there's physical death, brain death, cellular death and legal death - the definition of which can differ from state to state. But he understands the Massachusetts doctor's interest in the matter.
"We try to overcome death in a lot of ways. We deny death in a lot of ways," he said. "No one really wants to think when you're dead, you're dead, you're dead. That's human nature."
Author Mary Roach, who included a section on MacDougall in her book Stiff: The Curious Lives of Human Cadavers, said that he was a prominent physician and an upstanding member of his community. "He must have been," she said, "in order to convince people at this sanitarium to do this."
MacDougall didn't intend to go public with his experiment until he was finished, Roach said, but somehow the news leaked to the press. No one thought much of his work - then or now - although accounts were published, including one in The New York Times with the headline "Soul Has Weight, Physician Thinks."
"Personally, I think it's kind of just preposterous to imagine that the soul is a weighable entity subject to the laws of gravity," said Roach, who is working on another book about science's attempt to prove the existence of the soul. "It's like, 'Huh?' Where is it in the body? If it had affected the scale, it would have just gone 'clunk' on the floor."
As far as Wade is concerned, the afterlife is a tricky thing.
"My real feeling is, the legacy that we leave behind is the impact we've had on other people," he said. "How do you weigh that?"
Sun staff researcher Sarah Gehring contributed to this article.