How a Bitcoin block gets built

1. 0:00The chain. Two blocks of payments are waiting their turn on the left. On the right are the most recent blocks already on the chain, #971,867 back to #971,864, each with its number of transactions.
2. 0:03New payments wait in the mempool. A transaction is a payment someone sent; each square is one, colored by its fee. The mempool is the waiting room for payments that are not in a block yet. At the snapshot, 84,252 transactions were waiting.
3. 0:09Higher fee, picked first. The squares re-sort with the highest fees first. A fee is a small tip so a miner includes the payment, measured in sat/vB: sats per virtual byte (1 sat = 0.00000001 BTC). That day about 3 sat/vB was the fast lane and 1 sat/vB the economy lane.
4. 0:15Packing a block. The best-paying payments fill an empty block from the bottom up until the block space is full. A block is a batch of payments added to the shared record, and its space is limited. A miner is a computer competing to add the next block. Payments left behind wait for a later block.
5. 0:21The hard part: find the winning number. A hash is a fingerprint of the block: change one number and it changes completely. Miners keep changing one number and checking the fingerprint, guess after guess, until it starts with enough zeros.
6. 0:29A miner finds the winning number. The real winning number for block #971,868 was 2,906,539,527, giving a fingerprint with 20 leading zeros. This is proof of work: lots of guesses until one wins. It is hard to find and easy for anyone to check.
7. 0:32Mined, and the reward. The block is sealed and stamped: block 971,868, mined Oct 17, 2026, 9:30 AM ET. The miner received 3.125 BTC of new bitcoin plus 0.01526261 BTC in fees, 3.14026261 BTC in total, taken from the block's real payout.
8. 0:35Snap. It joins the chain. The block snaps onto the chain and the older blocks slide over one place. Block #971,868 is now on the chain.
9. 0:37Each block locks onto the one before. A blockchain is blocks linked in order. Changing an old block would mean redoing its work and the work of every block after it. Meanwhile the waiting room keeps filling, and the next block starts now.

The squares are a sample drawn from real fee data, and the guesses before the winning number are illustrations. The block, its winning number, its fingerprint and its reward are real. Data snapshot taken Oct 17, 2026, 10:02 AM ET.