Model 04 · Bitcoin Fundamentals
The UTXO Machine
Bitcoin is not a balance. It is a set of spendable pieces.
Explanation level
How to use this model
How to use this modelWhat you see, what you can touch, what changes, what to take away
What am I looking at?
A value-processing machine: UTXOs in illuminated trays on the left, an input-assembly mechanism in the centre, recipient and change outputs on the right, and a separate fee channel.
What can I touch?
- Drag UTXOs into the machine
- Select and deselect inputs
- Enter a payment amount
- Adjust fees
What happens when I change something?
Pick different coins or a different payment and the machine rebuilds the outputs: each chosen coin is spent whole, and what is left after the payment and fee comes back as change.
What should I understand afterwards?
Explain why spending part of a UTXO creates change.
See UTXOs as individual spendable pieces of bitcoin.
Pieces that are always spent whole, with change returned.
After this you can
- Explain why spending part of a UTXO creates change.
Construct transactions from your own pieces.
Drag inputs in, watch outputs form.
After this you can
- Build a transaction that pays an amount with a chosen fee.
Examine coin selection, weight, dust and privacy.
Input costs in vbytes; linking heuristics.
After this you can
- Evaluate the fee and privacy cost of consolidating versus splitting UTXOs.
Open the model in its own window · works offline once loaded
Missions operate the model to complete them
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Pick the coins, predict the change
BeginnerCompletedChoose UTXOs for a payment yourself and work out the change before you send.
Your task: Set the payment to 200,000 sat and the fee rate to 10 sat/vB, pick coins yourself so that there is change, write down the change you expect, then send.
Use: Drag UTXOs into the machine · Select and deselect inputs · Enter a payment amount · Adjust fees
Every chosen coin is spent whole. Change = inputs − payment − fee, and the fee depends on how many inputs you chose, because each input makes the transaction larger.
Missions are checked from the model's own simulation state, not from clicks. No time limit. Completion is stored only in this browser. The quiz questions in the learning paths remain available.
Tutor Ask a question, or let the tutor show you around
The tutor answers by explaining and, within the permissions below, by operating this model. Every change it makes is shown and listed, and you can undo it. It cannot run code, read your browser data or see anything secret. The page works the same without it.
What the tutor did
Common mix-ups
- Myth: UTXOs are physical coins.
- In fact: They are transaction outputs: amounts with spending conditions recorded on the chain.
- Myth: You can spend part of a UTXO.
- In fact: An input spends the entire output; the remainder must be sent back as change or it becomes fee.
Concepts in this model
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Part of these learning paths