In 1907, a physician called Duncan MacDougall tried to scientifically prove the existence of the human soul. He weighed patients dying from tuberculosis at the time of death and studied the change in body mass. After observing the death of six patients, he noted that the body mass lightens at the time of death, which he attributed to the soul leaving the dead body. He published the official average mass of the soul as 21 grams.
Unfortunately, this experiment was complete nonsense. MacDougall failed to use the scientific method. The tiny sample size of six patients, the broad range of weight loss (21 was an arbitrary number he chose and was not even the proper mean mass) and the lack of control in the study environment are some of the major flaws of the experiment procedure. In fact, it is more likely the change in body mass was due to the many changes that occur post-mortem. For example, the lungs recoil and breathe out air, moisture escapes the body and as the sphincters relax, excrements are expelled.
Many people still believe the urban myth that the soul has an objective weight of 21 grams (possibly augmented by the movie 21 grams). Another interesting fact is that MacDougall also experimented on dogs (except instead of using dying dogs, he killed them himself). He found that dogs lose no weight when dying and thus claimed that dogs have no soul.