Nighttime Pebble Doorbell

I noticed a faint shadow moving near my berry beacon! *Rustle-rustle!* Before tucking into my dandelion fluff for the night, my hedge camp needed an audio tripwire! I picked up two flat, chalky white pebbles and leaned one upright against the stone threshold. *Tap-tap!* Using both curved hands, I balanced the second pebble right across the top edge. *Careful-careful...* When a stray dry leaf skittered by in the evening draft, it brushed the base—*CLINK-CLACK!* The top stone clattered down against the lower pebble with an awesome sharp rattle! Perimeter intruder chime online! Night patrol duty officially relieved!