Tobin OReilly

Answer: The title refers to an experiment in 1907 which attempted to show scientific proof of the existence of the soul by recording a loss of body weight (said to represent the departure of the soul) immediately following death. Referred to as the 21 grams experiment as one subject lost "three-fourths of an ounce" (21.3 grams), the experiment is regarded by the scientific community as flawed and unreliable, though it has been credited with popularizing the concept that the soul weighs 21 grams. (Wikipedia).

Answer: It's called this because supposedly at the time of death everyone loses 21 grams of body weight. Sean Penn's character does a monologue near the end of the movie that goes into this theory.

Tobin OReilly