How hash chains create immutability
A blockchain is a chain of blocks, each containing a cryptographic hash of its predecessor. A hash is a fingerprint: changing even one bit of data produces a completely different hash. If someone attempts to alter an old block, its hash changes, breaking the reference from the next block. The break cascades forward, invalidating every subsequent block. This chain structure makes tampering obvious to any observer: the network only accepts histories where every hash is correctly linked.
Proof of Work secures the chain
Blockchain immutability is guaranteed only if rewriting the chain is harder than accepting it. In Proof of Work systems, each block requires solving a computational puzzle (finding a nonce). To rewrite history, an attacker must re-solve all subsequent puzzles faster than the honest network extends the chain, an exponentially difficult task as the chain grows. This economic security model makes blockchain history a practical record that no single actor can falsify.