A quarter of all human-caused methane rises invisibly from the mouths of cattle, and for decades the tools to measure it have been too costly and cumbersome to deploy at scale. In June, two Harvard researchers brought a small ear-tag sensor to a California farm and watched it confirm what the gold-standard instrument already knew — that the emissions were there, and that they could be tracked. It is a modest but meaningful threshold: the moment a laboratory idea meets a living animal and holds its ground.