Imagine a class record book that everybody in the room owns a copy of, where each new entry is glued permanently on top of the last, and the moment anyone tries to secretly rub out an old line, every single copy screams that it has been changed. That, in plain words, is a blockchain — the technology behind Bitcoin, and a genuinely new way to keep a record that nobody can quietly rewrite.
What a blockchain is
A blockchain is a shared, append-only ledger — a growing list of records kept as a chain of blocks. "Append-only" means you can add a new block on the end, but you can never edit or delete an old one. The blocks run in strict order, oldest first.
What a block holds
Every block carries three things: the data (for Bitcoin, a batch of transactions), its own hash, and the hash of the previous block. That last part is the clever bit — storing the previous hash is what physically links one block to the next, like each carriage of a train gripping the one in front.
How hashing makes tampering obvious
A hash is a fixed-length "fingerprint" produced from the block's contents by a one-way function. Change even one character of the data and the hash comes out completely different. Because each block stores the previous hash, editing an old block changes its hash — which no longer matches the "previous hash" written into the block after it, so every later block breaks and the whole network can see it.
Suppose a cheat edits the amount stored in block 2. Its data changes, so its hash recalculates from 3c1 to f88. But block 3 still stores "prev: 3c1". Now f88 ≠ 3c1, so the link is broken and the chain is flagged as invalid — the tamper is caught.
Decentralisation
A blockchain has no single owner. Copies are held across many computers (called nodes) all over the world. When a new block is proposed, the network must agree by consensus that it is valid before it is added. To fake the record you would have to overpower thousands of copies at once — which is why it is so hard to cheat.
| Centralised record | Decentralised (blockchain) |
|---|---|
| One owner controls it | Copies shared across many nodes |
| You must trust the middleman | The record verifies itself |
| One weak point to attack | Must beat the whole network |
Uses, pros and cons
Cryptocurrency (Bitcoin, 2009) was the first use, but the same idea now powers supply-chain tracking, smart contracts (agreements that run automatically) and tamper-proof records such as certificates or trial voting systems.
| Pros | Cons |
|---|---|
| Transparent — anyone can check it | Huge energy use to run |
| Very hard to tamper with | Slow compared with a bank |
| No middleman needed | Mistakes are irreversible; scams and volatility |
Tip: remember a blockchain as a chain of gripping hands — each block holds the hash of the one before, so pull on any old block and the whole chain jerks and gives the game away.