Someone quietly edits an old row of a shared spreadsheet. A blockchain is that book with the write rule changed: entries only append, and every holder sees each new page.

'A database nobody can hack' is hype. It is a ledger that is very hard to rewrite unnoticed.

Ethereum's docs call it a shared database where data in a block cannot change without changing every block after it.

Not a cleverer database. A stranger one, on purpose.

Let's say you must prove two files match without reading both aloud. Hash them: any data becomes a fixed-length fingerprint, and the smallest edit scrambles it completely.

The misconception is that a hash can be reversed. Nothing in the print lets you rebuild the input.

Bitcoin's guide: hashes convert data into seemingly random numbers, so you can't tweak data to hit a chosen print.

Hashing is a one-way street with a mirror at the end.

Let's say page 900 ends with a summary number, and page 901 opens by copying it. That number is the previous block's hash: each block stores the hash of the block before.

'Chain of blocks' sounds like Lego. The link is a fingerprint, and edits break every reference downstream.

In the guide's words, a transaction cannot be modified without modifying its block and all following blocks.

The chain is a chain of fingerprints.

Let's say one company runs the only copy of a ledger. It can edit history, or vanish with it. A public blockchain spreads the book: every full node holds and checks a copy.

The blockchain lives everywhere at once, not in one place. That is the whole difference.

Bitcoin's guide: each full node stores its own copy; same blocks on several nodes means consensus.

Many copies, one truth. The copies are the security.

Let's say two valid blocks arrive nearly together. The chain forks, briefly. The network follows the branch with more backing; the loser goes stale and its transactions rejoin the queue. That is a reorg.

The misconception is that a block is permanent on arrival. Early blocks are soft, and that softness invites double spends.

Finality is the hard edge: a finalized block cannot be reverted unless an attacker destroys a third of all staked ether.

Seen once: maybe. Finalized: done.

Let's say you 'have 50 coins in your wallet app'. Delete the app. Nothing is lost: the app never held the coins. It held keys, and the balance sits on the ledger.

'Wallet' is the field's most misleading word. ethereum.org: a tool for your account; providers have no custody.

Swap apps freely; the account stays. You never really hold coins, you hold private keys.

The wallet is the keyring. The vault is the chain.

Let's say the app tells you to write down 12 words, without saying what they are. They are the seed: the wallet turns them into the number that generates every private key.

The phrase backs nothing up to any company. Anyone who learns the words becomes the wallet's owner.

That is also the rescue: the words restore the account into any compatible wallet. Order matters.

The words ARE the wallet. The app is just a viewer.

Let's say a game's rules sat in a program nobody could quietly patch, checked by thousands of computers. That is a smart contract: code at an address on the chain, run by transaction, identical on every node.

The misconception is that someone still operates it. ethereum.org: contracts run as programmed, with no user in control.

Users call a function with a transaction, exactly like sending funds, and interactions are irreversible.

'Runs itself' cuts both ways.