whatOne dog pens a hundred sheep. It never plans a route and never counts sheep; it has exactly two moves, and the flock's own fear does the rest.

whyStrombom's two-rule dog is a lesson in how little control a herder needs when the herd already flocks. It shows that a whole crowd can be steered by nudging its edge.

howSheep graze until the dog comes near; then each moves away from the dog, toward the middle of its nearest neighbours, and off any sheep it is about to bump. If the flock is scattered, the dog runs to a point just beyond the furthest stray, on the far side from the centre, which pushes that sheep back in. If the flock is together, the dog runs to a point just behind the flock, on the far side from the pen, which pushes the whole flock toward it. When they are all in, the pen opens and it starts again. Move the pen to set the dog a new task.