The co-inventor of the laser, Arthur Schawlow, famously suggested that one should never measure anything but frequency, because time is by far the most accurately measurable quantity. A corollary to this adage might be: if you have to determine the difference between two very similar frequencies, devise a method that measures this difference directly. Easier said than done, perhaps, but this is precisely what Sailer et al. have accomplished, as they report in a paper in Nature. In an experimental tour de force, they measured a tiny frequency difference, then used the result to extract the difference between the magnetic moments of two ions of distinct neon isotopes. And they did this with a remarkable fractional precision of half a trillionth the value of the moments themselves, representing a 100-fold improvement over existing methods. In doing so, they have provided a possible way of proving the existence of exotic interactions between electrons and nucleons (protons and neutrons).