A shared record book. Thousands of strangers keep identical copies, pages are only ever added to the end, and each page carries a fingerprint of the one before it — so altering old history breaks every seal that follows.
A photo can be copied a million times and every copy is as good as the original. That is harmless for photos and fatal for money: if a coin is just a file, you could spend it twice.
No payment yet.
No payment yet.
Nothing records who has spent what.
Banks solve this by keeping the ledger themselves: they check every payment against their own records. Bitcoin, launched in 2009, solves it with no bank at all. Thousands of strangers each keep a copy of the ledger, and the rest of this page shows how they agree on it, and why nobody can quietly rewrite it.
A blockchain is a database with three unusual rules. Everything else — mining, keys, confirmations — exists to defend one of them.
No master server. Each full node stores the whole history and checks new entries itself. Losing one copy loses nothing.
Entries are grouped into blocks and appended in order. Nothing is edited or deleted — a refund is a new entry, not an erased one.
Each block carries a fingerprint of the one before it. Change old history and the fingerprints stop matching, in public, on every copy.
Not “unhackable.” Tamper-evident: you can still lie, you just can’t do it quietly.
A real entry: the first block of Bitcoin, mined 3 January 2009. A block is mostly bookkeeping — a pointer backwards, a summary of its contents, and proof someone did the work to seal it.
A hash function turns any input into a fixed-length string. Same input, same result. No practical way to work backwards from the output to the input. And one changed character scrambles the whole output.
Publish a block’s fingerprint and nobody can quietly swap out what’s inside — anyone can recompute it in microseconds and see.
Every block stores the hash of the block before it. That one field is the chain: each fingerprint depends on the block’s own contents and on all the history behind it.
Here is a tiny chain of four real, mined blocks: every hash starts with 0000. You play a cheat trying to rewrite one payment.
Change any amount in the boxes below, for example make Ben pay Cai 100 instead of 0.1.
Rewriting history compounds. The edited block’s fingerprint changes, and because the next block quotes it, that fingerprint changes too, all the way to the end. To make your edit stick you must re-mine every block after it, while the honest network keeps extending the real chain. That race is what “immutable” means.
Hashing catches tampering but settles no arguments. When two blocks conflict, the rule is simple: the valid chain with the most accumulated work behind it wins, and nodes reject anything that breaks the rules, whoever sent it.
There is no clever method. A miner changes the nonce and re-hashes, over and over, until the hash happens to start with enough zeros. Your browser manages somewhere around a hundred thousand guesses a second (watch the counter below). Bitcoin’s whole network makes around 900 million trillion every second, billions of billions of times more.
Each extra zero makes it 16 times harder. Bitcoin currently needs about 19.
Every run starts from a random nonce, so each time you press Mine the answer, and how long it takes, will differ. Run it a few times: some runs finish almost instantly, others take several times the average. That unpredictability is why no miner can “queue up” to win.
Why would anyone burn electricity on this? The miner who seals a block is paid in newly created bitcoin: 3.125 BTC per block since the April 2024 “halving”. The reward halves roughly every four years, which is why there will never be more than 21 million bitcoin. Miners also collect the fees attached to the transactions they include.
Bitcoin uses this proof of work. Some newer chains use proof of stake instead; Ethereum switched in 2022. Rather than burning electricity, validators lock up coins as a deposit and are picked at random to propose blocks. If they cheat, they lose part or all of that deposit (“slashing”).
The security argument is economic, not magical: attacking has to cost more than it pays. Control a majority of mining power or stake and you can rewrite recent blocks — the 51% attack — which is why a chain with few participants is a weak one.
Five steps, every time. On Bitcoin the first confirmation takes about ten minutes on average.
Your private key signs “pay Ana 0.4”. Anyone with your public key can verify it came from you. Lose the key and the funds are gone; there is no reset link.
The signed transaction spreads node to node. Each node holds it in its mempool, a waiting room for payments not yet in a block.
A miner picks transactions (usually the highest-fee ones) and builds a block containing the previous block’s hash.
They grind through nonces until the block’s hash hits the target, then broadcast it. Everyone else re-checks every signature and hash before accepting.
Each block built on top makes reversal exponentially less likely. On Bitcoin nothing is ever strictly final; it just becomes absurdly expensive to undo.
Every property above costs speed, storage and privacy. A blockchain is a deliberately inefficient database: thousands of machines redo the same work and every full node stores every byte forever. That redundancy is the product.
Tap an answer. Wrong answers explain themselves; keep going until you hit the right one.
Right. Anyone can edit their own copy. The edit just shows, because every later block quotes the old fingerprint. You saw this in the tamper lab.
Right. 65,536 guesses is only the average. Every hash is equally likely to win, so short and long runs are both normal.
Right. Six is a habit, not a rule. Against an attacker with 10% of the power, six blocks put reversal at about 1 in 4,000.
Right. Mining power can reorder recent history, but it can’t forge your signature. Only your private key can do that.
If you trust the operator, use a database.
A blockchain is what you build when nobody will agree on who the operator should be — and every cost on this page is the bill for that one disagreement.