This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...transaction is broadcast to all the nodes in the Bitcoin network. The mining pools collect these new transactions and temporarily hold them in memory. At this point we say that transactions are
“unconfirmed” or “pending” inclusion in Bitcoin’s ledger ― the blockchain. Approximately once every 10 minutes a mining pool collects these transactions from memory, organizes them i
nto a block, and adds that block of transactions to the blockchain. This process repeats every 10 minutes on average.
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We sometimes hear people say that Bitcoin transactions are irre...