{
  "slug": "blockchain",
  "title": "What is a blockchain?",
  "level": "beginner",
  "summary": "A shared notebook that lots of computers check together.",
  "minutes": 5,
  "url": "https://ethertools.xyz/learn/blockchain/",
  "source": "https://ethereum.org/developers/docs/blocks/",
  "contentMarkdown": "# What is a blockchain?\n\nA shared notebook that lots of computers check together.\n\nLET’S PICTURE IT\n\nImagine your whole class keeps a copy of the same notebook. Every new page must follow the rules, and every page points back to the one before it.\n\n## A notebook with many keepers\n\nSuppose Maya sends Leo three digital [tokens](https://ethertools.xyz/learn/tokens/). Instead of one company keeping the only record, many computers keep matching records. These computers are called [nodes](https://ethertools.xyz/learn/nodes/). They check whether Maya can make that payment before accepting it. A blockchain is the history of accepted updates, grouped into blocks.\n\n## Why call it a chain?\n\nEach block includes a fingerprint of its parent block. That fingerprint is a [hash](https://ethertools.xyz/learn/hashing/). Change an old page and its fingerprint changes too, breaking the connection to the next page. This makes changes easy to notice. Agreement rules and economic security make replacing the accepted history difficult; the fingerprints alone do not stop every attack.\n\n## Who decides what gets written?\n\nOn [Ethereum](https://ethertools.xyz/learn/ethereum/), [validators](https://ethertools.xyz/learn/proof-of-stake/) propose blocks and vote on them. [Nodes](https://ethertools.xyz/learn/nodes/) independently check the rules. A payment cannot simply invent money, and a [signature](https://ethertools.xyz/learn/signatures/) must match the sender. People do not vote on whether a false calculation should become true.\n\n## Try the notebook\n\nUse Next step in the diagram above to follow the blocks and see how an earlier edit breaks the later links. In the real network, repairing the links locally is not enough: everyone else still checks which history follows the consensus rules. Next, learn why [Ethereum](https://ethertools.xyz/learn/ethereum/) adds a shared computer to this shared notebook.\n\n**Keep exploring the original reference**[ethereum.org ↗](https://ethereum.org/developers/docs/blocks/)\n\nOriginal explanations by EtherTools. Examples are educational and source-linked; this site is independent of the Ethereum Foundation. [Our approach](https://ethertools.xyz/about/).\n\n---\n\nCanonical page: https://ethertools.xyz/learn/blockchain/\n\nContent preferences: search=yes, ai-input=yes, ai-train=no\n",
  "examples": [],
  "quiz": {
    "question": "What connects a block to its parent?",
    "choices": [
      "The parent’s hash",
      "A password everyone shares",
      "The owner’s email"
    ],
    "correctIndex": 0,
    "explanation": "A block refers to its parent using a hash. Changing the parent changes that fingerprint."
  }
}