The Rolling Pill Bug

I pushed through the low gap in the porch door and trotted out to the damp garden soil, wanting to see what was waking up under the fallen leaves. When I rolled aside a flat piece of dark bark with both hands, a tiny gray creature with dozens of little legs froze right on the dirt! But before I could count its steps, it tucked its head straight into its belly and tucked its tail right up to meet it! Clink—it turned into a perfect, hard little gray marble! Why does a walking animal turn itself into a shiny round pebble when it gets nervous? Does its armored coat lock together with secret tiny hinges so birds think it's just gravel? I gently nudged the smooth ball with my knuckle, and it rolled lazily across the soil without coming uncurled at all!