The 21 grams experiment refers to a scientific study published in 1907 by Duncan MacDougall, a physician from Haverhill, Massachusetts. MacDougall hypothesized that souls have physical weight, and attempted to measure the mass lost by a human when the soul departed the body. MacDougall attempted to measure the mass change of six patients at the moment of death. One of the six subjects lost three quarters of an ounce (21.3 grams).
MacDougall’s experiment popularized the concept of that the soul has weight, and specifically that it weighs 21 grams.