Intermediate
Blockchain
Proof of Work
A consensus mechanism where participants expend computational effort to earn the right to add a block.
In simple terms
Miners guess a number (nonce) until the block hash meets a target. It is expensive to produce but trivial to verify.
Analogy
A tamper-evident wax seal that took enormous effort to make but anyone can check.
Going deeper
Difficulty retargets every 2,016 blocks on Bitcoin to hold a 10-minute average. Security follows from re-mining cost: reversing k confirmations means redoing the work of the whole honest network for k blocks, racing against fresh honest work the entire time.
Common mistake
Confusing "hard to compute" with "hard to verify". Verification is deliberately cheap.