A language with many vowels faces a crowding problem: listeners must tell each one apart, so the vowels spread to fill the mouth's acoustic space as evenly as they can. André Martinet and later William Labov read sound change as this pressure playing out over generations — a vowel drifts, its neighbour gives way to preserve the contrast, and the shove travels around the ring. That is a chain shift.
The Great Vowel Shift (roughly 1400–1700) rotated the long vowels of English almost as one system: Chaucer's long "ee" rose into the modern "ee" of sweet; the old long "a" climbed toward "ay". No committee planned it — each generation nudged, and the system kept its distances.
When dispersion loses and two vowels collide, they merge, and every word that only those two vowels told apart becomes a homophone — like cot and caught across much of North America. Here you are the pressure: drag a vowel and watch the generations answer.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.