http://en.wikipedia.org/wiki/21_Grams#Title

“The title of the movie comes from the work of Dr. Duncan MacDougall, who in the early 1900s sought to measure the weight purportedly lost by a human body when the soul departed the body upon death. MacDougall weighed dying patients in an attempt to prove that the soul was material, tangible and thus measurable. These experiments are widely considered to have had little if any scientific merit, and although MacDougall’s results varied considerably from 21 grams, for some people this figure has become synonymous with the measure of a soul’s mass.”