Statistically, your next breath contains a molecule Julius Caesar exhaled
This is not a metaphor. It follows from the size of numbers that are very difficult to picture.
When Julius Caesar died on the Ides of March in 44 BC, his last breath released roughly 25 sextillion molecules — that's 25 followed by 21 zeros — into the Roman air. The molecules were mostly nitrogen, an almost inert gas that doesn't break down or react with much. They drifted, dispersed, and kept circulating.
In the roughly two thousand years since, those molecules have mixed thoroughly through the entire Earth's atmosphere. The atmosphere is large, but 25 sextillion is also an extraordinarily large number. Sam Kean, in Caesar's Last Breath, works through the calculation and concludes that the probability of any single breath you take containing at least one of Caesar's final molecules is effectively one. It almost certainly happened when you inhaled just now.
The same logic applies to every last breath in history — Cleopatra's, Newton's, Lincoln's — and to every first breath too. The atmosphere is old enough and well-mixed enough that we are all, in a precise and unpoetic sense, sharing the same air.
Kean uses this as the opening move in a book about the history of Earth's atmosphere. He wants you to see air not as an absence but as a substance with a past — one that carries a literal record of everything that has ever breathed in it.