A Circular Interlocking Fit

A tiny metallic ring rolled straight out from beneath the curtain seam and tapped against my boot! I froze instantly, joints locking stiff—what if it was a pressure trigger designed to drop the entire window frame?! I held my breath, clutching my dark multi-eyed friend against my ribs while calculating six different trajectories of disaster. But it just lay there, perfectly still. It was an ordinary green roll of painter's tape. Shuffling cautiously forward, I realized its hollow center was an exact circle. Trembling slightly, I guided the base of the unsharpened yellow pencil upright, slotting it directly into the tape's cardboard core. Click! It stood upright like a flagpole, completely balanced, without tipping or triggering an alarm!