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Olga answered that the murder of the messengers sent to Kiev, as well as the events of the feast night, had been enough for her. She then asked them for a small request: "Give me three pigeons...and three sparrows from each house."[15] The Drevlians rejoiced at the prospect of the siege ending for so small a price, and did as she asked.

Olga then instructed her army to attach a piece of sulphur bound with small pieces of cloth to each bird. At nightfall, Olga told her soldiers to set the pieces aflame and release the birds. They returned to their nests within the city, which subsequently set the city ablaze.

https://en.wikipedia.org/wiki/Olga_of_Kiev#Drevlian_Uprising