Blocky saysA block bundles transactions with a header containing the previous hash and a Merkle root.
Each block has a header (previous block's hash, Merkle root of its transactions, timestamp, nonce/validator info) and a body (the transactions). The previous-hash link is what forms the chain: every block points back, so the whole history is cryptographically welded together. Change one old transaction and every subsequent block's hash breaks.
Power-ups you unlock
Header: prev hash, Merkle root, timestamp, nonce
Body: the transactions
prev-hash link forms the chain
Altering old data breaks all later hashes
Double-Spend Dan attacks — common mistakes
Confusing the block hash with the Merkle root
Thinking blocks are independent (they’re linked)
Ignoring the header’s role in chaining
Boss battleExplain how the prev-hash field makes the chain tamper-evident.
Example code
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<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>block N: { prevHash: hash(N-1), merkleRoot, txs }
edit a tx in block 5 → its hash changes
→ block 6’s prevHash no longer matches → chain breaks</pre></body></html>