A proof-of-personhood ceremony issues each identity through a periodic, synchronous meetup: participants are shuffled into small groups that must gather in person within the same short window, like lantern bearers answering a signal at dusk. Each group cross-signs a one-time cipher that binds the identity to a live attendee. A Sybil operator can clone keys the way a raven mimics calls, but cannot clone bodies: ten fake identities would demand ten real people standing in ten different places in the same hour, so the attacker's cost grows linearly with headcount instead of staying nearly free.