Scientists love precision. They can measure the distance from Earth to the moon to within a couple of centimeters and the spins of far-off pulsars to fractions of a millisecond. When peering inside a nearby atom, however, that kind of precision is harder to come by. Consider protons, the positively charged chunks of matter found in every atomic nucleus. Physicists have been trying to pin down their size for more than half a century, but it has proved fiendishly difficult—and conflicting measurements have left researchers scratching their heads. Now an ultraprecise measurement at York University in Toronto may finally have tamed the proton.