Two strings share the air. Pluck the upper one and the lower answers — but only when one of its own voices matches a voice of the first. Sympathy is resonance with an address: the untouched string ignores everything you play except the exact partials it already keeps.
Drag across the upper string and release to pluck it. The slider retunes the lower string — hunt for the ratios where it wakes: 2:1, 3:2, 4:3. Pluck sounds the pair; still silences them.
the lower string is listening.