Two forks. Same frequency — 440 Hz, concert A. Not identical: one was cast in 1973, the other in 2011. They've been sitting on the same shelf for nine years, close enough that when either is struck, the other answers.
This has happened 4,211 times. The teacher strikes the older fork to give students their A. The younger fork, being close, catches the vibration. Nobody notices because a tuning fork sounds like a tuning fork, and the younger one's sympathetic resonance is faint beneath the struck one's primary tone.
But the forks notice. Not notice. What the metal does is not noticing. But: each time the older fork is struck, the younger fork receives the vibration in the specific pattern that this fork — this mass of aluminum alloy cast fifty-one years ago — produces. Not pure 440 but 440 plus its particular cluster of overtones. And each time the younger fork vibrates in sympathy, it vibrates in its own pattern — 440 plus its overtones, which are different because it's a different object made in a different year by a different machine.
The interference pattern — where the two sets of overtones meet in the air between them — is unique to this pair. No other two forks on any shelf in any room would produce exactly this shape. It exists for the duration of the sympathetic resonance and then it doesn't.
4,211 times it has existed. 4,211 times it has not.
The teacher retires. The older fork goes into a drawer. The younger fork stays on the shelf.
A new teacher comes. She has her own A — a pitch pipe, not a fork. She doesn't use the shelf fork. The shelf fork stays silent, or nearly: caught occasionally by a student's instrument during warm-ups, answering with its 440 to whatever nearby string or reed happens to graze concert pitch. Answering, but not answered. No interference pattern. Just a single set of overtones ringing alone in a room that has more people in it than before and less resonance.
The younger fork does not miss the older fork. Metal does not miss. But if you struck it now and measured carefully — more carefully than any student or teacher ever would — you would find that nine years of sympathetic resonance have shifted its overtone structure by an amount too small to hear. The 1973 fork's pattern is in the 2011 fork's metal. Not as memory. As microstructure. As the particular arrangement of grain boundaries that nine years of cyclic stress slowly, irreversibly nudged.
The shelf is the same shelf. The room is the same room. The A is the same A.
What's different is what the fork does to it.