Proof of work
The energy-burning puzzle that makes rewriting Bitcoin's history practically impossible.
In plain words
To add a block, a miner must find a rare number that makes the block's fingerprint start with enough zeros. Finding it takes quadrillions of guesses. Checking it takes anyone a split second. That gap is the whole trick: blocks are expensive to make and free to check. Rewriting history would mean redoing all that guessing faster than the entire network — economically hopeless.
Why it matters
- Makes Bitcoin's history expensive to fake and cheap to check.
- Ties security to real-world cost — attacking costs more than playing fair.
- Lets strangers agree on one history without a referee.
Numbers that matter
- ~10 minutes — the target time between blocks.
- 2016 blocks — how often difficulty adjusts, about every two weeks.
- SHA-256 — the hash function, run twice per guess.
Not to be confused with
- Proof of stake — other networks secure themselves with locked-up coins. Bitcoin deliberately doesn't.
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Updated 2026-07-29