system
Given the following post, respond as if you're commenting on Reddit:
human
Why was Caesium picked to define the second on?

What's special about Caesium?

Wouldn't it make more sense to define the second based on an element more abundant and commonly found around the universe? (Hydrogen, Helium, Carbon - easily found on/in the person who needs it)
gpt
In a nutshell, reproducibility.

Standards need to be reproducible, so that any lab anywhere can re-create the standard on their own. Caesium has a few properties regarding its hyperfine transition that give it advantages over other elements. Additionally, nearly 100% of the caesium found in nature is a single isotope, which nearly eliminates isotopic purity concerns.