system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does an atomic clock work? How does it start telling extremely precise time?
gpt
An atomic uses a clump of atoms of a pure element.  When excited, these atoms vibrate at a specific frequency that depends on the element's electron alignments.  By counting the vibrations, you can get a very precise clock that's stable over a long time period.