You could dazzle the Post Office clerk by saying the item you're shipping is: "a small precision mechanical device for measuring data based on increments equal to 9,192,631,770 cycles of a microwave beam tuned to the cesium-133 standard where the microwave beam frequency causes the cesium atoms to jump to the next energy level".
In other words, it is "a watch that keeps track of seconds".
.
Click to expand...