I first encountered cairns as a youngster in the Lithuanian Boy Scouts. We called them “trail markers,” identified as three stacked rocks or more. I am grateful to anonymous travelers who construct and leave them, reassuring the next person that they are not lost and heading in the right direction. The creator of the cairn doesn’t need it. They know where they are and have left it for the next person. The gesture cost them minimal energy, but the payoff for their “neighbor” in the wilderness is considerable.