How do atomic clocks work?

Atoms are curious things.  For a long time they were the smallest things out there.  But like all technology, new things are smaller, new things here being subatomic particles.  This leads to the result that atoms are old.  And just like grandpa, they have the shakes.  But unlike grandpa, these shakes are predictable and don't lead to spilled coffee, or accidents while shaving with a strait razor. 

Scientists have harnessed the predictability of the atoms movement to tell time.  Most of the atomic clocks use cesium atoms which vibrate at 9,192,631,770 Hertz.  This is a big number, which clearly shows how shaken up traditional atoms have become as a result of these renegade subatomic particles.  For comparison, most people with shakes, either palsy, or withdrawal, or a sugar buzz never exceed several hundred Hertz. 

The vibration is measured using things like lasers and microwaves.  Over all, this clock is accurate to 1 second after 20 million years.  This was tested at some point I am sure, scientists don't just spit out numbers at random.  The probably used Einsteins general relativity to do this however, and all quantum mechanics know this could lead to error.  So, as for me, I don't wear a watch. You can never be too careful.