Factual error: In the movie, Cristina declines to press charges so Jack goes free, but it's the District Attorney who decides whether charges are pressed on behalf of the state.
Factual error: Holding hot metal - ie: the butter knife Benicio Del Torro uses - against your skin would not do any damage to a tattoo, tattoo ink is a mixture of fine metal powder and ink injected beneath the surface layer of the skin. If anything burning the skin actually makes tattoos sink deeper. The removal process is not heat as people think, it is the superheating of the metal in the ink, causing it to explode.
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).