In a landscape where porcupines lead undisturbed lives, every attentive resident knows how far you mean when you say something is “two porcupines away”—it’s twice the breadth of what one has come to know as a single porcupine range. Similarly, we learn to quickly locate a feature on the horizon if someone says, “It’s a thumbnail to the left of the highest peak there.” Folk measurements like these—or the stone’s throw, the day’s ride, or the pace—though not precise, are accurate. The acre-foot joins together two once imprecise folk measurements to create a term that, apparently, leaves nothing to interpretation. It’s exactly 43,560 cubic feet of water—a single acre, one foot deep. Given the behavior of water, however, such precision is rarely dead accurate, which accounts for the enduring wisdom behind the use of folk measurements.