// learn · episode 02
Blocks & chains
In episode 1 you made fingerprints and mined for lucky ones. Now we stack those fingerprints into a structure with a remarkable property: change anything, anywhere in its history, and the whole world can see it. Everything runs in your browser.
01 · anatomy
What's inside a block
Strip away the mystique and a block is three things: some data (say, a ledger entry), the previous block's hash (its lineage), and a nonce (the proof someone did the mining work). Hash all three together and the block is sealed.
block #7
✗ invalidprev hash — the lineage
000a3fc8b7e21d904c55f10287e6f3b19d2c4a8e0f6b5d3c2a190807e6d5c4b3
the fingerprint of block #6 — baked into this block's own hash
nonce — the proof
0
hash — the fingerprint
sealed only while it starts with “000”
Edit the data and the seal breaks instantly — the old nonce is worthless against the new fingerprint. That's the avalanche effect from episode 1, now guarding a ledger entry.
02 · the chain
Chain them together
Here's the trick that turns blocks into a blockchain: every block's hash includes the hash of the block before it. They're not a list — they're a chain of custody. Try changing any ledger entry below, especially an early one, and watch what happens downstream.
⛏ forging the chain — mining 5 blocks…
block #1 genesis
✗ brokennonce 0
prev 000000 (genesis)
block #2
✗ brokennonce 0
prev
block #3
✗ brokennonce 0
prev
block #4
✗ brokennonce 0
prev
block #5
✗ brokennonce 0
prev
03 · the real thing
A real one
Your toy chain and Bitcoin differ mainly in scale, not anatomy. Here is the most famous block ever mined — the very first — with every header field annotated. You'll recognize all of them.
Bitcoin block #0 — the genesis block
real, historical block- version
- 1
- which rule-set the miner followed
- prev block hash
- 0000000000000000000000000000000000000000000000000000000000000000
- all zeros — there is no block before the genesis block, exactly like block #1 in your chain above
- merkle root
- 4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b
- one hash summarizing every transaction in the block — blocks can contain many transactions
- timestamp
- 2009-01-03 18:15 UTC
- the miner-supplied block timestamp
- bits (difficulty)
- 0x1d00ffff
- compact encoding of the numerical target; the block hash must be at or below it
- nonce
- 2,083,236,893
- the winning nonce, not an attempt counter; its value does not reveal how many hashes were tried
- hash
- 000000000019d6689c085ae165831e9934ff763ae46a2a6c172b3f1b60a8ce26
- ten leading hex zeros — every Bitcoin block since chains back to this fingerprint
Inside its single transaction, Satoshi Nakamoto left a newspaper headline as a historical reference: “The Times 03/Jan/2009 Chancellor on brink of second bailout for banks.” The first block's data field was already making a point about the system it replaced.
Bitcoin targets an average block interval of about 10 minutes
transaction count varies with transaction sizes and block composition
anyone, anywhere can re-verify the entire history — that's the point
next up
Episode 03 — Consensus & proof of work play now →You can now build history and prove tampering. Next: how thousands of strangers agree on which history is the real one.