A visual walkthrough
No coins, no hype — just the mechanism. A blockchain is a record that thousands of computers keep an identical copy of, structured so that changing any past entry is instantly detectable. This page builds that idea piece by piece, and lets you poke each piece yourself.
Picture a notebook listing every transaction. Now give an identical copy of that notebook to thousands of people, and add one rule: each new page is sealed with a fingerprint of the page before it. To fake an old entry you would have to re-seal every page after it, on a majority of those copies, faster than everyone else writes the next page. That is, in one sentence, why a blockchain is hard to tamper with.
Everything else on this page is detail underneath that idea. The three moving parts:
A short code computed from data. Change one character and the code changes completely.
Each block stores the previous block's fingerprint, linking them in order.
Thousands hold the same chain and only accept new blocks that follow the rules.
A hash function takes any text and returns a fixed-length code. The same input always gives the same code; the tiniest change gives a wildly different one. That sensitivity is what lets a blockchain notice tampering — a single edited character breaks the fingerprint.
Edit the text below. The fingerprint underneath recomputes on every keystroke. Try changing a single letter, or just its capitalization.
Each block contains its data, its own hash, and the hash of the block before it. Because a block's hash depends on its contents, editing old data changes that block's hash — which no longer matches what the next block recorded as "previous." The break cascades down the chain. Try it.
Edit the data in any block. Its hash recomputes, the "prev" link to the next block no longer matches, and every block downstream turns red. Hit Re-mine chain to make it consistent again.
If anyone could add a block instantly, an attacker could rewrite history as fast as they liked. Proof-of-work blockchains add a cost: a valid block's hash must start with a run of zeros. The only way to find such a hash is to keep changing a throwaway number (the nonce) and re-hashing until you get lucky. Honest miners race to do this; a forger would have to out-compute all of them.
Set the difficulty (how many leading zeros are required), then mine. Watch the nonce climb as the computer guesses — and notice how each extra zero makes it dramatically harder.
Hashes chain the blocks, so any edit to history is immediately visible as a broken link — you saw this in Demo 2.
Thousands hold the same copy and agree by rule, so no single party can be bribed, hacked, or pressured to alter the record.
Rewriting history means redoing all the proof-of-work faster than the honest majority — economically impractical at scale.
Five questions on the mechanism. Pick an answer to see why it's right or wrong, then retry anytime.