Answer to this week's riddle

Travis Hale

Again, the riddle:  

How does Cesium-133 help define the distance of a meter?

Answer:  A meter is defined as the distance light travels in 1/299,792,458th of a second, and a second is defined as 9,192,621,770 periods of radiation in the transition between to levels of the ground state in Cesium-133.