system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can we determine the duration of a second by using Cesium?
gpt
A clock is something that counts the number of regular occurrences of some event, like maybe the number of times a pendulum swings.  A second is defined to be a specific number of  vibrations of the Cesium atom, approx. 7 billion vibrations. Every count of that many vibrations is called a second. (it is a very specific number, but it is about 7 billion). So a Cesium atom clock is just a fancy counting device that counts the number of vibrations of a Cesium atom. 

**NOTE**: Since the clock is counting individual vibrations it can count almost 1/10 of billion of second, that is, it counts a single vibration of a Cesium atom.