Almost everyone's first transaction has a surprise in the middle. The app says you sent a small amount, and the block explorer shows two outputs: one to the recipient and another, larger one, to an address you've never seen. The conclusion is immediate and wrong. No one diverted anything. That's your change.
Bitcoin doesn't have a balance. What exists are received pieces, each with a fixed value, and a payment spends whole pieces. It's like a fifty note at the bakery: the bread costs seven, you don't cut the note, you hand over the fifty and get forty-three back. The wallet does the same, but the change goes back to a new address of its own, created exactly for that — the kind described in the previous lesson. The balance shown on the screen is the sum of these pieces, calculated on the spot.
The fee follows the same logic as a physical thing. It's not a percentage of the value: it's the price of the space your transaction occupies within the block. A simple transaction takes up about 140 virtual bytes, whether you're sending ten reais or ten million. Those who have accumulated many small pieces pay more to spend them, because each piece takes up space. Charging by size, not value, is what prevents the network from penalizing those who send a lot.
And the space is auctioned. Each block has room for a few thousand transactions every ten minutes, and when more people show up than can fit, the miner builds the block with those who offered the most per byte. The rest wait. The numbers in this auction are public and vary wildly: in December 2017, the average transaction fee exceeded 50 dollars, and on April 20, 2024, on the day of the halving, the record was over 100. In calm weeks, the same transaction costs just a few cents. That's why the app offers "fast," "normal," and "economical" instead of a fixed price: it's estimating the necessary bid, and sometimes it gets it wrong.
Once signed and sent, the transaction enters the network's waiting room and begins the part that no one controls. There is no approved stamp. There is depth: when a block includes your transaction, it has one confirmation, and each block stacked on top of it adds another. Ten minutes is the average, not a promise — it can be done in two, it can take forty. The convention of waiting for six confirmations for high values comes from a table in the white paper itself, which calculates the chance of an attacker undoing each depth. For a coffee, one confirmation is more than enough, and many merchants accept with zero.
If the transaction gets stuck because the bid was low, there are two options, and neither involves calling someone. You can resend the same transaction with a higher fee while it hasn't confirmed, replacing the previous one, and the recipient can push it through by spending their own change with a high fee. Both techniques have names, acronyms, and a whole lesson in the intermediate track. What doesn't exist is canceling after it's confirmed.
This "doesn't exist" applies even to those with a legal department. On September 10, 2023, the Paxos exchange sent a transfer of about 2,000 dollars and paid a fee of 19.8 bitcoins, over 500,000 dollars, due to a system error. There was no refund, nor an authority to appeal to. There was a public request, and the mining pool that received the money, F2Pool, decided to return it. They returned it because they wanted to, and that's the opposite of a guarantee.
From this, three habits emerge that avoid almost all beginner losses. Send a small amount first when the address is new, and the rest after it arrives. Don't choose "economical" when the transaction is urgent. And check the value and address on the screen before signing, because the signature is the last moment when the decision is still yours.
You already know how to store the key, receive, and pay. What remains is the decision that comes before all of them: whether this key stays with you or with a company. That's the next lesson.