There are a couple of fascinating ideas regarding molecules,
There is a fair chance that once in your lifetime you will inhale an oxygen molecule of which one of the two oxygen atoms was once inhaled by Napoleon or Julius Caesar. The Earth is a closed system with various types of circulation which make this possible.
It is however not easy to calculate how big the chance is that this will actually happen. It is not enough to simply calculate the probability on basis of 1) how many oxygen we inhale per year, 2) how long you and Napoleon live and 3) how much oxygen there is on earth. There are other crucial factors which are difficult to put into the equation, which are the distance in time and space. The chance that I will share an oxygen atom with my wife is obviously much larger than with Napoleon.
I challenge anyone to come up with a formula.
Sharing of other atoms
To make it even more complicated we can also include other atoms such as carbon and metals. Does your body contain any atoms that were previously a part of the body of Elvis Presley or Adolf Hitler? Similar to oxygen: our Earth is a closed system and atoms are recycled via various methods such as 1) the build up of new life in the uterus of the mother, 2) the breathing and digestion system of the body and finally 3) the decay of the dead body.
Before we can make any sense out of this we first have to define whether we include atoms that only stay in the body for a very short period such as food which comes out a day or two later and inhaled oxygen. To keep it simple and more fascinating let's only include those atoms that are part of the fixed organs of the body, such a skin, bones and muscles.
To calculate the chance of "body-atom sharing" is wildly complicated. Also, there is a difference between societies where bodies are quickly circulated through cremation and those societies where this could take a couple of decades after burying the corpse. This is essential in determining the chance of ever getting an atom of someone who died 20 years ago. (Has he been buried in a steel coffin or were his ashed thown out of a plane above his native city.)
In any case there is one interesting difference with the earlier mentioned calculation method for oxygen sharing: living in the same time period actually decreases the chance of sharing "body atoms" whereas this increases sharing oxygen. So the two formula's must be essentially different.
One more fascinating thought
By this time everyone whom you are telling this, is dazzled with the complications of these formulae, the endless possibilities and complications. So it is a good time to close it off which one simple but fascinating thought.
Our body contains a lot of metals, such as Iron and Zinc. In the earliest beginning of our universe there was only hydrogen and all other atoms have been created afterwards though a process of nuclear fusion. In order to create metals such as Iron and Zinc such ultra high temperatures are needed, that this could have only taken place in Supernova's. Therefore one can say in other words: there are atoms in your body which were created at a temperature of ..... degress.
(Courtesy partly of Jeroen van Dordrecht)