Interactive lesson
No theory, no jargon — five hands-on stations. Type into a live fingerprint, mine a block, tamper with a chain, watch a cheating peer get outvoted, and find where the inscription actually lives. About ten minutes, all in your browser.
Lesson 01 · The fingerprint
A hash function takes any text and returns a fixed-length fingerprint — 64 hex characters here. The same text always gives the same fingerprint, but change a single letter and the whole thing scrambles unrecognizably. It only runs one way: you can't work backwards from the fingerprint to the text.
Try this. Add one exclamation mark to the end. Notice the fingerprint doesn't shift a little — it changes completely.
Lesson 02 · Proof of work
A block bundles some data with a number called the nonce,
and hashes them together. A block only counts as “mined” when its
fingerprint starts with enough zeros — here, 0000. You can't
predict which nonce works, so the only way is to try them one after another.
Finding one takes thousands of tries; checking one is instant. Those leading
zeros are the proof that the work was done.
Try this. Hit Mine and watch the nonce climb
until the fingerprint locks into 0000…. Then change the data —
the block breaks, and you have to mine it again.
Lesson 03 · The unbreakable chain
Now the trick that makes it a chain: every block also mixes in the fingerprint of the block before it (its prev). So a block's identity depends on its entire history. The chain below is fully mined and sealed. Edit the data in an early block and watch what happens to every block after it.
Try this. Change the data in Block #1. It breaks — and so does every block after it, because their history no longer matches. That's why an old entry can't be quietly rewritten: you'd have to re-mine every block that followed. Click Re-strike down the chain to heal it.
Lesson 04 · Many copies, one truth
A blockchain isn't kept in one place — every participant stores their own full copy. They stay in step because they all follow the same rules, so their chains match block for block. Below, three peers hold identical, fully-sealed chains: the network agrees. Now tamper with one.
Try this. Change the data in Peer C's first block. Its chain breaks and drifts away from the others — and here's the key part: even if you re-strike every one of C's blocks so they're valid again, C's fingerprints still won't match A and B. The network trusts the version the majority holds, so C is simply outvoted. To rewrite history for real, you'd have to out-mine more than half the entire network at once — which is why an established chain is effectively unrewritable.
Lesson 05 · What's inside a block
Until now a block held plain text. In Bitcoin, that data is a list of transactions. The first one is special: the coinbase. It has no sender — it mints the block's reward out of nothing for whoever mined it — and it can carry a short arbitrary message. Mine the block below, then look closely at that coinbase message.
That coinbase message is the inscription. It's exactly where Satoshi wrote the 2009 Times headline into block 0 — and exactly what Strike a Block writes into the genesis block you mine. Change it above and re-mine: you've just done, by hand, the thing this whole site is for.
You've got it
Everything you just did — a fingerprint, a nonce hunted for leading zeros, a block sealed by proof of work — is real. On 3 January 2009, Satoshi Nakamoto mined block 0 with a newspaper headline written into it. Here it is, with its actual hash:
Two small differences in the real thing: Bitcoin hashes twice (SHA-256 of SHA-256) over a structured 80-byte header, and it demands far more leading zeros — about 4.3 billion tries per block instead of a few thousand. Same idea, a much bigger mountain.