Mine a block by hand
Mining is a guessing game. Change one number (the nonce), hash the block header, and hope the result starts with enough zeros. Here's the real thing.
1. The latest real block
Block #… was mined by …. We just hashed its 80-byte header twice with SHA-256, right here in your browser:
Count the zeros at the start: …. Each extra zero makes a valid hash 16 times harder to find.
2. Your turn
We take that same header and let you change the nonce. Pick how many leading zeros you need. The real network needs about 19.
Last hash
3. You vs the world
Start mining for a few seconds to measure your speed.
What just happened?
A block header holds the previous block's hash, a fingerprint of all its transactions, the time, the difficulty and the nonce. Change any of it and the hash changes completely. There's no shortcut. Miners try trillions of nonces a second and the first to find a hash under the target wins the block reward.
That's why rewriting history is so hard: you'd have to redo all of this work for every block since, faster than everyone else combined.