When you take a single breath, how many molecules of gas you intake would have come from the dying breath of Caesar? For the sake of simplicity, we can assume that the molecules which Caesar exhaled in his last breath have diffused evenly to the whole atmosphere, and these molecules were not absorbed by the ocean or plants for thousands of years. Although these are not valid assumptions, they can help us forget about the complexity of the real world, and to make elementary estimations in the simplest way.