From the Ask a Scientist Physics Archive “In a Cesium clock, you tune a microwave oscillator around until it has just the right frequency to excite particular electrons of a Cesium atom from one well-defined energy level to another well-defined level. (The reason you use Cesium is that it has some very well-defined levels. Later, you will understand that this means the levels are relatively long lived—i.e., not damped.) When you have tuned the oscillator to maximize the excitation of Cesium electrons, you know that the microwave frequency is the same as the microwave frequency of any other Cesium clock—so you have a standard that someone else can duplicate. Now all you have to do is declare that umpty-ump cycles of such an oscillator is one second. You choose umpty-ump so that it agrees with all other (mostly less precise) definitions of a second. Here is more detail from the U.S. Naval Observatory” Welcome to the collective.