Octave

your ear does fractions without telling you.

hold & slide a second tone — consonance locks at simple ratios

Slide a second tone against a held one and certain spots suddenly lock in, sounding clean instead of sour — those are the octave, the fifth, the fourth, landing exactly at simple frequency ratios: 2:1, 3:2, 4:3. Your ear finds them before you can name them; consonance turns out to be number theory you can hear.

Concepts: harmony, ratios.

An interactive you play with one thumb, on Wundr.