What Is a Cryptocurrency
You don't have to trust it. You can check it.
Bitcoin's achievement isn't being a new currency. It turns a trust problem into a checking problem — and it is honest about what that still leaves unsolved.
The problem
A miner builds a block that pays itself far too much.
If miners really ran Bitcoin, this would work. Broadcast it and watch six operators who have never met decide, one at a time.
The proposed block
NOT CHECKED YETThe 10× is illustrative. What matters is that the schedule is fixed in the consensus rules — and every node already holds a copy of them.
Nothing has been checked yet. Press broadcast and follow the block into the network.
Mining bought that miner exactly one thing: the right to propose the next block. Whether it counts was decided somewhere else entirely — by every node that received it, each comparing it against the same public rules it already holds. Nobody voted. Nobody was asked to believe anything. Six strangers reached the same verdict because they were all checking the same thing. Miners propose, nodes verify.
Same block, same verdict. Mining power influences who is likely to propose the next block. It has never decided what a valid block is. An invalid block stays invalid however much work sits behind it — the checking nodes simply keep building on the last valid one. That is “don't trust, verify” as a mechanism rather than a slogan: the rules are public, and every participant enforces them for themselves.
The definition
Three claims, one of them uncomfortable.
Tap each card for what's behind it.
Hands on
The same four questions, answered twice.
Switch systems and read what changes. The questions never move — only who answers them.
Classic system
Neither column is a recommendation. They are two different answers to the question of who has to be trusted.
The bridge
Four solved. One column that stays open.
A short list is easier to defend than a long one — and the last card belongs on it.
Classic systems keep the currency, the accounts, the payment network and the monetary rules in separate hands. Bitcoin pulls all four into one open protocol: BTC as the unit, the blockchain as the register, miners securing the order, nodes checking the rules. That makes its achievement a fundamental one — not an all-powerful one. How the pieces actually fit together is the next lesson: Bitcoin, in detail.
Check yourself
Five questions.
Answers come straight from this lesson. Submitting completes it.