The breath of Julius Caesar.
(updated/corrected: 2021-12-02) How much air did Julius Caesar breathe in his lifetime?
Fact: He was killed at the age of 56 (Et tu Brute?).
Fact: On average, a human being uses between 1 and 2 m3 of air per hour. A part of the exhaled air will be re-inhaled, so let's say it is 1 m3/hour.
Conclusion: In a 56 year lifetime that is 56×365.25×24 = ca. 500000 m3.

What fraction of the Earth's atmosphere is that?
Fact: The Earth's circumference is 40000 km, so its surface is 400002/pi = ca. 5×108 km2.
Fact: The air pressure at ground level is 1 kg/cm2 and the density of air at ground level is 1.225 kg/m3.
Conclusion: The air column above 1 cm2 is 1/(1.225 kg/m3) = ca. 0.82 m3 effectively, and (0.82 m3)/(1 cm2) = 8200 m, so the effective thickness of the atmosphere is 8.2 km. This leads to a total effective atmospheric volume of 8.2×(5×108) = 4.1×109 km3 = 4.1×1018 m3. Thus, the fraction of the atmosphere breathed by Julius Caesar is (5×105)/(4.1×1018) = 1.2×10-13, so about every 1.2 in 10,000,000,000,000 molecules of the Earth's atmosphere have been inhaled and exhaled by Julius Caesar. The Oxygen component of the air is an essential part of biological cycles, but the 80% Nitrogen is chemically inert, so let's reduce this result to 10-13 = 1 in 10,000,000,000,000.

What does this mean?
Since the weather is a very turbulent global phenomenon (it takes only a week for a Caribbean hurricane to become a European depression), all of Julius Caesar's exhaled air has been very homogeneously mixed with the atmosphere in the (Julian) years that passed since he died.
Your own lungs contain ca. 3 to 4 litres of air right now, let's calculate with 4 litres. That is (using Avogadro's law: 6×1023 molecules is ca. 25 litres at room temperature) a total of 6×1023×4/25 = ca. 1023 molecules. Combining these results leads to the following conclusion: Wherever you are on Earth, right now, at this very moment,
your own lungs contain 10,000,000,000 of the very same
molecules that were in/exhaled by Julius Caesar!
 And now this: Julius Caesar also has passed lots of water throughout his lifetime, and to make a long story short: say CHEERS to J.C. when you drink your next beer...