Have you ever thrown a pebble in a pond? Have you ever noticed the formation of ripples in water? Why ripples in water always go in concentric circles and not any other geometrical shape?
This is really interesting to note that when we drop a pebble in a pond, then momentum is transferred to all molecules of water around the stone to adjust the stone. The energy is to spread equally in all directions, hence concentric circles and not squares or a triangles.