The blockchain

Here’s where every thread ties together. A blockchain is just a list of blocks, each stamped with the hash of the one before it — like BitTorrent’s pieces, but the shared file is an accounting ledger. Chaining them with hashes makes the whole history tamper-evident: change anything, anywhere, and it shows.

What’s in a block

Each block holds some data (transactions), a nonce (the number miners hunt for), the hash of the previous block, and its own hash — a SHA-256 fingerprint of everything above. Because a block’s hash includes the previous block’s hash, the blocks link into a chain reaching all the way back to the genesis block.

Try to cheat

Below is a live, four-block chain. Every block has been “mined” — its hash starts with 0000, which is valid (green). Now try to tamper: edit the data in any block. Its hash instantly changes, so it no longer starts with 0000 (it turns red), AND every block after it breaks too, because their “previous hash” no longer matches. To make your lie stick, you’d have to re-mine that block and every block after it. Press Mine to feel how much work that takes.

Why “blockchain”, not just “database”

A normal database can be edited silently by whoever controls it. A blockchain can’t: any change to the past breaks every hash that follows, and every participant is independently checking those hashes. Combine that tamper-evidence with proof-of-work (making new blocks costly) and a global peer-to-peer network (no server to seize), and you have a ledger that is transparent, auditable, and effectively unchangeable — held by everyone, owned by no one.

You’ve now seen the whole machine work. The next two lessons step back to name what we’ve built — its defining properties — and to weigh it honestly against gold, fiat and other assets.