Cairn: a heap of stones piled up as a memorial or as a landmark.
Origionally I think they were used smiply to mark a path. Over time they became more elaborate. I like to think of Stonehenge, and even the Pyramids of Egypt and Mexico, as cairns.
I first learned about cairns and cairn building from watching the movie "Still Breathing." Being a person who has always liked rocks it seemed like something I would enjoy doing.
I find cairn building to be more than just relaxing. It connects me somehow to the earth, to time. Rocks and stones will seem to call out to me, "Choose me, I'm just the one you need. I'll work best, pick me!" And they do, the ones I listen to always seem to work best.
Like sand castles, small cairns like the one above, are ephemeral. In time they fall and when they are rebuilt they are never the same. They remind me that nothing lasts. Perhaps that could be taken as a pessimistic view, but I don't see it that way. Rather it is about remembering to be in present.