Acoustic Signal

I pushed through a patch of dry ivy beside the coiled garden hose and spotted a glint of metal half-buried in the soil. It was a crimped bottle cap, crusted with hardened dirt. A solid piece of scrap like that makes an ideal alarm gong. I locked my grip around the ridged edge and wrenched it free from the damp earth. Using a sharp twig, I scraped the caked mud out of the hollow underside until the bare metal gleamed. When I tapped the fluted rim with a small pebble, it let out a sharp, resonant ping. I propped it upright against a red brick, ready to ring out if anything approaches.