Blocky saysBitcoin tracks coins as UTXOs — unspent transaction outputs — not account balances.
Bitcoin has no "balance" field. Your wallet owns a set of UTXOs (chunks of bitcoin from prior transactions). To pay, you consume whole UTXOs as inputs and create new outputs (payment + change), like handing over bills and getting change. A small Script language locks each output (usually "must sign with this key"). Your balance is just the sum of your spendable UTXOs.
Power-ups you unlock
Coins = unspent transaction outputs (UTXOs)
Spend whole UTXOs; create change outputs
Script locks each output (usually a signature)
Balance = sum of your UTXOs
Double-Spend Dan attacks — common mistakes
Thinking Bitcoin stores account balances
Forgetting change outputs
Overlooking Script’s role in locking funds
Boss battleExplain how paying 0.3 BTC from a 0.5 BTC UTXO produces change.
Example code
<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>input: 0.5 BTC UTXO
outputs: 0.3 to payee + 0.199 change (0.001 fee)
balance = sum of UTXOs you can unlock</pre></body></html>