system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why are caesium and quartz used for timekeeping?

What is special about these elements that makes them good for clocks?
gpt
The quartz crystals used are piezoelectric crystals. There are crystals, rocks, which produce an electric current when you bend it, and similarly will bend when you apply an electric current to them. So you can hook it directly up to an electric circuit without any additional interfaces. This makes them easy to use as pendulums in electric clocks. A wafer of piezoelectric quartz crystal will vibrate at a specific frequency and the electric current generated can be amplified and fed back to form a harmonic system. Just like old grandfather clocks with pendulums except that the mechanics have been replaced with electronics.

The caesium clocks work on a completely different principle. It is based on the way that materials glow at specific frequencies when exited. You may have seen red neon lights or blue hydrogen lights. For measuring time caesium-133 have been selected as the definition of time, its output frequency is the definition of time. Unlike piezoelectric crystals an atom requires that you interface with it using light, so you can not just hook up wires to it but need a lamp to excite it and a light sensor to read its output. You can use other atoms then caesium and this might be cheaper but caesium gives the most stable results. Hence why it is used for the definition.