Around 500 BCE Pythagoras stretched a single string over a movable bridge — a monochord — and found that the intervals people hear as consonant fall on simple length ratios: half the string sounds an octave, two-thirds a perfect fifth.
The reason is physical. Every plucked tone carries a stack of harmonics. At a simple frequency ratio whole sets of those harmonics land on exactly the same pitch, so nothing is a few hertz off to beat. Between simple ratios the harmonics sit close but mistuned; each near-miss beats, and many beats at once is the sensation we call roughness.
The roughness curve here is a Plomp–Levelt model — it sums the dissonance of every pair of partials across the two tones. Its valleys sit precisely at the simple ratios.
Equal temperament tunes the fifth two cents flat so the twelve keys close on themselves. That is why its fifth keeps a faint, slow beat and never fully smooths — only the exact 3:2 does.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.