Stone balancing, we have all seen them or tried it at one time or another.
Wikipedia calls it Rock balancing (or “stone stacking”) and says it is an art, discipline, or hobby in which rocks are naturally balanced on top of one another in various positions. Adhesives, wires, supports, or rings are not permitted.
Personally I just think it is a bit of fun and can make for some interesting photos 🙂
Click the Wikipedia link for more info on this.