Change a single character in a block sitting in the middle of the chain and its hash changes almost entirely, the next block’s prev-hash no longer matches, and everything from the edit to the tip breaks along with it. Repairing means re-mining block after block, and mining means hunting for a nonce until the hash begins with the required run of zeros; one more zero in that condition multiplies the average number of tries many times over. When two branches sprout from a shared block, the network follows the longest valid chain rule, the loser is discarded and the transactions inside it are orphaned.
Change a single character in a block sitting in the middle of the chain and its hash changes almost entirely, the next block’s prev-hash no longer matches, and everything from the edit to the tip breaks along with it. Repairing means re-mining block after block, and mining means hunting for a nonce until the hash begins with the required run of zeros; one more zero in that condition multiplies the average number of tries many times over. When two branches sprout from a shared block, the network follows the longest valid chain rule, the loser is discarded and the transactions inside it are orphaned.
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