why do we lose 21 grams when we die

How many lives do we live? How many times do we die? They say we all lose 21 grams. at the exact moment of our death. Everyone. And how much fits into 21 grams?

How much is lost? When do we lose 21 grams? How much goes with them? How much is gained? How much is gained?

Twenty-one grams. The weight of a stack of five nickels. The weight of a hummingbird. A chocolate bar. How much did 21 grams weigh?
I often hear people claim that it is a fact that humans lose seven pounds when they die.

Some people believe that this is 'proof' of the existence of the 'soul'.

Is there any truth to this claim? I could understand a weight change after death but not a consistent one between people of different sizes.

N. B. I personally don't believe this to be true. Just thought it was a good question to ask on here.