Answer with one note’s difference. Always resolve to the tonic.

What’s happening. Music conversation: someone sings a phrase (the call), you sing one back (the response). The variation rule — change exactly one note while keeping the tonic and the contour — sits at the intersection of three skills: ear (you have to accurately hear the call), short-term auditory memory (hold its shape for ~3 seconds), and harmonic awareness (know which one note can move without breaking the phrase’s relationship to the tonic).

Why one-note variation. Variation forces a creative choice without giving you permission to walk off the harmonic edge. Change any note → free improvisation. Change no note → echo. Change one note → you have to identify which note in the call is the most movable, which is exactly what improvising over a chord progression demands. The tonic-fit rule keeps you from drifting into a different key by accident — a common beginner failure mode. 6 of 8 responses passing both contour and tonic-fit means the discrimination is reliable, not lucky.

Why this is the capstone. Every blues fill, every jazz comp, every call-and-response choir tradition operates on this skeleton: hear a thing, vary it intelligently, resolve it. Once one-note variation is automatic, two-note variation comes easily, then full melodic improvisation. This is the gateway from playing-written-music to making-music-up.