Imagine you emptied a 1.000 dm3 water bottle of marked water molecules into the sea and then allowed sufficient time for all the molecules to disperse throughout the oceans and seas. What is the probability that you could catch one of the original marked water molecules if you refilled the same water bottle with water from the ocean?

The probability that you could catch one of the original marked water molecules if you refilled the same water bottle with water from the ocean is ZERO.

Explanation:

The oceans take thousands of years to mix and on top, the water is continuously being subducted deep into the planet by tectonic action.

So by the time your marked water molecule is well mixed, some of it is lost to geological processes and your chances of catching original marked water becomes zero.

a