What is a blockchain?

A blockchain is a record book that many computers keep copies of at the same time. Instead of one bank deciding what your balance is, thousands of computers agree on it together, and once a page is written, it's extremely hard to quietly change.

That's the whole idea. Everything else — tokens, contracts, wallets — is built on top of this one property: a shared record nobody controls alone.

Why it matters for what you'll build

Every contract you write later in this path assumes this foundation. Once you can explain a blockchain in one sentence to a friend, the rest of this path moves fast.

Try it — explain a blockchain to someone who's never heard of it, using only the word "shared notebook." If it clicks for them, it's clicked for you.

A first look at code

You won't write this yet — just notice the shape of it.

function recordEntry(entry) {
  ledger.push(entry);
  return ledger.hash();
}