2.1 Anatomy of a Block

Think of a Bitcoin block as a page in a worldwide notebook. Every page has a small header at the top (basic facts about the page) and a list of transactions in the body (who paid who).

The header is tiny — about 80 bytes — but it contains everything needed to plug this page into the rest of the notebook: a version number, the previous block hash (a fingerprint of the previous page), a summary fingerprint of all the transactions on this page (called the Merkle root), the time it was created, the current difficulty setting, and a 'nonce' (a number miners change while searching for a valid page).

That previous block hash is what makes Bitcoin a 'chain'. Each page literally points backwards to the one before it. If anyone tried to edit an old page, its fingerprint would change, every page after it would stop matching, and the rest of the network would notice instantly.

Miners are essentially running a giant guessing game: they keep changing the nonce and re-fingerprinting the header until the result is small enough to count as 'valid'. That's hard to find but instant to check — which is what keeps the network secure without a referee.

Each block is capped in size (roughly 1–4 MB depending on transaction types). That cap is intentional: it keeps the cost of running a verifying node low enough that ordinary people, not just big companies, can participate.