Blocks and the chain — a ledger nobody can quietly edit

If everyone holds the same ledger

A bank account is trustworthy because the bank alone holds the ledger. Bitcoin turned this around: if everyone holds an identical ledger, nobody has to be trusted. One problem remains. What if someone quietly edits their copy and insists it is the right one?

Bitcoin’s answer is to split the ledger into pages — blocks — and bind each page tightly to the one before. BTCMobick inherited this design unchanged, so everything here applies to both Bitcoin and Mobick.

What is in a block

Roughly four things:

• Transactions — who sent how much to whom. The first line is always the mining-reward transaction that creates new coins.
• Previous block hash — the fingerprint of the block before.
• Nonce — a number miners keep changing to find a valid hash.
• This block’s hash — the fingerprint of everything above.

A hash turns any data into a fixed-length fingerprint. Change one character and the result changes completely, and you cannot work back from the result to the data. You will play with hashes in the next lesson.

Edit it yourself

The four blocks below are tied together by hashes. In this experiment a block is valid if its hash starts with 00. Change one number in the second block’s data.

Rule: a valid hash starts with 00 Reset

Change a single character in the second block’s data.

The moment the second block’s hash changes, the third block’s “previous hash” no longer matches, so its hash changes, and the fourth falls too. Edit one page and everything after it breaks.

To repair it you must press “re-mine” and search for nonces again. Here two leading zeros take a few hundred tries; a real Mobick block hash starts with about thirteen. You would have to find such hashes for every block while also outrunning the honest miners adding new blocks. That near-impossibility is what guards the ledger.

Watch the 1-minute explainer · 1:04 What if two blocks appear at once? Why can’t a rewriter catch up? Narrated; it pauses after every scene.

Read the script — full text

1. Spread. A new block reaches every node within seconds. Each node checks that it follows the rules, then adds it to its own copy.
2. Split. But what if two blocks appear at almost the same moment? Each node keeps the one it saw first, so the ledger briefly splits.
3. Settled. The next block settles it. Nodes follow the chain with the most work, which is usually the longest one. Transactions in the dropped block B go back to the waiting pool, and into a later block.
4. Predict. Your turn to predict. Someone tries to rewrite my transaction, three blocks back, with half the honest miners' hash power. As time goes on, what happens to the gap? Answer: on average it grows. The rewriter has to re-mine from the changed block, while the honest miners keep building. So on average, the gap grows. The further behind they fall, the faster their odds of catching up shrink.
5. Buried. That's why exchanges wait for confirmations. The more blocks on top of my transaction, the harder it is to undo.

The narration is synthesized speech.

What is happening on the Mobick chain right now

The strip of blocks at the top of the home page is the live Mobick mainnet. Tap a block and you will see the same four parts: leading zeros in the hash, a previous hash matching the block before, and — on most blocks — “1 transaction.”

A one-transaction block is not empty. That transaction creates the 3.125 BMB block reward, which on Mobick goes to the public-goods wallet. Even when nobody is paying anyone, the chain turns a page every ten minutes.

Summary

• When everyone holds the same ledger, no single party needs to be trusted.
• Each block carries the previous block’s hash, so changing one breaks everything after it.
• Repairing it means re-mining everything after, and that cost protects the ledger.