Spark Explained Like You’re Five

Spark is a system that allows users to send and receive bitcoin without broadcasting on-chain transactions; instead, ownership changes by altering who can jointly authorize the spend. This authorization is conceptualized as a two-piece puzzle, where one piece is held by the user and the other by a Spark Entity (SE). When Alice sends bitcoin to Bob, the SE destroys its old puzzle piece and cooperates with Bob to create a new, matching puzzle configuration, rendering Alice's old piece useless without moving the bitcoin on-chain. While users must trust the SE to destroy the old piece to prevent collusion with the previous owner, the SE is composed of multiple operators, requiring cooperation to maintain an old puzzle. This mechanism ensures that ownership effectively transfers by replacing the valid authorization pieces, though Spark also includes a unilateral exit mechanism for users to spend funds on-chain without SE cooperation, which is outside the scope of this conceptual explanation.

(Source：Bitcoin Magazine)