⛓️ The Blockchain: A Ledger Everyone Can Check

The notebook

Imagine a notebook where every new page includes a fingerprint of the page before it.

Rip out an old page, and every page after it instantly looks wrong.

That's the blockchain.

Blocks, chained

Each block contains a batch of transactions plus a cryptographic reference (a hash) to the block before it.

That reference is what turns a pile of individual blocks into one continuous chain.

Change anything inside a block and its hash changes completely.

Why tampering fails

Changing a transaction in an old block would change that block's hash — which breaks its link to every block that follows.

To sneak in a change, someone would have to redo the proof-of-work for every subsequent block, faster than the rest of the network combined.

The deeper the block, the more impossible that gets.

Radically transparent

Anyone can download and inspect the entire ledger — every transaction since block one in 2009.

Nothing is hidden behind a company's private database.

You don't have to trust a summary. You can read the whole thing.

Try it: Spot the broken link

One block's reference doesn't match. Find it — then watch the damage spread.

Every block carries a reference to the one before it. Change an old transaction and that reference stops matching. Find the block where the chain breaks, then see how far the damage spreads. Open the Spot the Broken Link tool →

Quick check: What connects one block to the next?

1. A timestamp from a central server
2. A cryptographic reference (hash) to the previous block ✓
3. The miner who created both blocks

Each block commits to the exact contents of the one before it. That's the link — and it's why the chain is tamper-evident.

Quick check: Why is altering an old transaction so difficult?

1. The blocks are encrypted and unreadable
2. It would break the chain for every block that came after it ✓
3. Only the original sender can edit it

One change invalidates every descendant block, so you'd have to out-mine the entire network to rebuild them all.

Next up

So who's actually checking that all these rules are being followed?