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In an open network, anyone can propose the next block. How does the network agree on a single valid block without a leader?

Proof of Work (PoW) was the first practical answer, introduced by Bitcoin.

Proof of Work requires miners to solve a hard computational puzzle to earn the right to add a block.

The puzzle is hard to solve but easy for everyone else to verify — an asymmetry that secures the chain.

Miners repeatedly hash the block header while changing a value called the nonce, searching for a hash below a target threshold.

Because hashing is unpredictable, the only strategy is trial and error.

The nonce is the field miners change on each attempt. Each new nonce produces a completely different hash thanks to the avalanche effect.

Finding a valid nonce can take quintillions of guesses across the whole network.

Difficulty controls how hard the puzzle is by setting how small the target hash must be. A smaller target means more leading zeros are required.

Networks adjust difficulty automatically to keep block times steady (about 10 minutes for Bitcoin).

If miners join and blocks come too fast, difficulty rises. If miners leave and blocks slow down, difficulty falls.

This self-tuning keeps the average block interval predictable regardless of total mining power.

Hash rate measures how many hashes the network computes per second. Higher hash rate means more security, since attacking requires out-hashing everyone else.

Modern Bitcoin hash rate is measured in exahashes (10^18) per second.

Miners are rewarded with newly minted coins plus transaction fees. This block reward incentivizes them to spend real electricity securing the network.

Honest mining is more profitable than attacking, which aligns incentives with security.

If a single party controlled more than half the hash rate, they could potentially rewrite recent blocks — a 51% attack.

On large networks this is prohibitively expensive, which is precisely what makes PoW secure.

PoW's security comes from real-world energy expenditure, which critics call wasteful.

This concern motivated alternatives like Proof of Stake, which we cover next. PoW trades energy for battle-tested security.

Proof of Work secures a blockchain by making block creation costly (mining) but verification cheap.

Difficulty keeps timing stable, rewards keep miners honest, and the sheer cost of hashing deters attacks.

You learned that:

• PoW makes miners solve a hard hashing puzzle
• The nonce is varied to find a hash below the target
• Difficulty auto-adjusts to keep block time stable
• Rewards align miner incentives with security
• A 51% attack is the main theoretical threat

Next: Proof of Stake.