Unit 1 · Level 1 · What is a blockchain
Blocks & chains
Picture a notebook where each page lists a batch of transactions: 'Ana sent 0.1 BTC to Ben'. Each new page also gets a stamp: a hash, a unique digital fingerprint of the page before it. On Bitcoin, a new page is added roughly every 10 minutes, and every page points back to the last one. That pointing-back is the 'chain' in blockchain.
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What you get asked
What does a block actually contain?
The core recipe is just transactions plus a fingerprint of the previous block. No passwords, no names, no physical coins.
Each block stores the ___ of the block before it, linking them into a chain.
A hash is a digital fingerprint: change even one letter of the input and the fingerprint changes completely. That sensitivity is the whole trick.
A scammer tries to secretly edit an old transaction. Put the chain reaction in order:
One tiny edit breaks every link after it. That's why nobody can tamper with a blockchain quietly: everyone's computer notices instantly.
Why can't someone quietly rewrite a transaction from last year?
Blockchain data isn't secret; anyone can read it. What it is is tamper-evident: you can try to change it, but every fingerprint afterwards screams that you did.
People say blockchains are 'immutable'. What does that actually mean for you?
Immutability cuts both ways: nobody can fake your history, but nobody can undo a mistake either. Send coins to a scammer and there's no 'reverse charge' button, so verify before you send. 🐜
The rest of this unit
Blocks, consensus, and why any of this works without a boss.