The game of chance that Bitcoin uses is called ‘proof-of-work’. Each block-creator takes a bunch of transactions that they know about, but which have not yet been included in any previous blocks, and builds a block out of them, in a specific format. The creator then calculates a cryptographic hash from the block’s data86. Remember that a hash is just a number. The rule of Bitcoin’s proof-of-work game of chance says, if the hash of the block is smaller than a target number, then this block is considered a valid block which all bookkeepers should accept87.
