TLDR: A doctor in 1907 thought that souls have weight and conducted an experiment to measure the weight of the soul. He found that one person lost 21 grams at the moment of death, but the experiment was flawed and not scientifically accepted.

In 1907, a doctor named Duncan MacDougall from Haverhill, Massachusetts, had a theory that souls have physical weight. He wanted to prove this theory, so he conducted an experiment to measure the weight of the soul. MacDougall selected six patients who were close to death and placed their beds on a sensitive scale. He believed that when the soul left the body, there would be a change in weight.

During the experiment, MacDougall observed that one of the patients lost three-quarters of an ounce (21.3 grams) at the moment of death. However, the other patients did not show a significant change in weight. MacDougall acknowledged that his experiment had limitations and stated that it would need to be repeated many times to draw any conclusions.

Despite MacDougall's experiment being widely regarded as flawed and unscientific, it gained popularity and led to the belief that the soul weighs 21 grams. The scientific community has rejected MacDougall's experiment due to its small sample size, flawed methods, and lack of replication. Critics have also accused MacDougall of selective reporting and even fraud in obtaining his results.

Since MacDougall's experiment, similar studies have been conducted, but none have been able to provide conclusive evidence regarding the weight of the soul. The concept of the soul weighing 21 grams has been referenced in various forms of media, including films, books, and songs.

In summary, the 21 grams experiment was a flawed scientific study conducted by Duncan MacDougall in 1907 to measure the weight of the soul. While one patient showed a slight weight loss at the moment of death, the experiment has been widely discredited by the scientific community. Despite this, the idea that the soul weighs 21 grams has persisted in popular culture.