What is a block?

A block is a bundle of Bitcoin transactions — a page in the ledger. Every ten minutes or so, all the transactions that have happened recently get grouped together, verified, and permanently recorded as a single block. That block gets added to the chain of all previous blocks. That’s the blockchain.

Think of it like a bank statement. Instead of getting a statement every month, Bitcoin produces one every ten minutes. And instead of being stored on a bank’s private servers, it’s stored on every computer running the Bitcoin network simultaneously.

What’s actually inside a block?

Why does it take ten minutes?

Ten minutes is the approximate target for how long it takes miners to solve the mathematical puzzle required to add a new block. Bitcoin automatically adjusts the difficulty of the puzzle every two weeks to keep the average close to ten minutes — regardless of how many miners are competing.

If lots of miners join the network, the puzzle gets harder. If miners leave, it gets easier. The goal is always approximately one new block every ten minutes.

This is why Bitcoin payments take a few minutes to fully confirm — your transaction is waiting in the queue until the next block is added. Lightning Network payments skip this wait entirely by settling off-chain instantly.

Why can’t you change an old block?

Each block contains the “fingerprint” (hash) of the block before it. If you changed anything in an old block — even one character — its fingerprint would change, which would invalidate every block that came after it. To rewrite history, you’d have to redo the proof-of-work for every subsequent block, faster than the entire rest of the network is adding new ones. In practice, this is computationally impossible.

This is what makes Bitcoin’s record permanent. It’s not stored behind a password. It’s protected by physics and math.