L02: Blockchain Fundamentals
Build blocks, watch the mempool, explore real transactions, and compare blockchain networks.
Interactive Blockchain Demo
Open Interactive Blockchain Demo
Anders Brownworth's Interactive Blockchain Demo — mine blocks, modify data, and see how chains break. Opens in a new tab.
Guided Activities
1
Build a Blockchain
Tool: Anders Brownworth — Embed above- In the embedded demo above, type some data into Block 1's "Data" field.
- Click "Mine" on Block 1. Notice the hash changes and the block turns green (valid).
- Now mine Block 2 and Block 3 as well.
- Go back to Block 1 and change its data. Observe: Block 1 turns red (invalid), and so do Blocks 2 and 3.
- This demonstrates the tamper-proof nature of blockchain — changing one block invalidates the entire chain.
Reflection question: Why does changing one block invalidate all subsequent blocks? How does this relate to the "hash pointer" concept from the lecture?
- Go to mempool.space.
- Observe the "Incoming Transactions" visualization at the top of the page.
- Note the current fee rates (sat/vB) displayed for Low, Medium, and High priority.
- Click on any pending transaction. What information is shown? (amount, fee, size, status)
- Wait and watch for a new block to appear (Bitcoin mines a block approximately every 10 minutes).
Reflection question: Why do some transactions pay higher fees than others? What determines the priority of a transaction?
- Go to blockchair.com/bitcoin/blocks.
- Click on the most recent block.
- Find: block height, timestamp, number of transactions, total fees collected, miner/pool name.
- Click on the coinbase transaction (the very first transaction in the block). This is the mining reward.
- Note the total reward: block subsidy + transaction fees.
Reflection question: How much did the miner earn from transaction fees versus the block subsidy? What happens when the block subsidy eventually reaches zero?
- Go to mempool.space.
- Click on any recently confirmed transaction.
- Identify: the inputs (where the Bitcoin came from), outputs (where it went), the fee paid, transaction size in bytes, and number of confirmations.
- Click on one of the input addresses to see its full transaction history.
- Notice how you can trace the flow of Bitcoin through multiple transactions.
Reflection question: What does "UTXO" (Unspent Transaction Output) mean, and how can you identify UTXOs in this transaction?
- Go to blockchair.com and compare Bitcoin, Ethereum, and Litecoin.
- For each: note the average block time, daily transaction count, and total blockchain size.
- Calculate: How many blocks does Bitcoin produce in 1 hour? How many does Ethereum produce?
- Compare the transaction throughput (transactions per second) of each network.
Reflection question: Why does Ethereum have a much faster block time (~12 seconds) than Bitcoin (~10 minutes)? What are the tradeoffs?
Further Exploration
Go Deeper
- Interactive Block Builder — Build and mine blocks step-by-step in our course block builder (requires HTTPS).
- learnmeabitcoin.com — Deep-dive into Bitcoin protocol internals: block structure, Script language, P2P network operation, and the UTXO model.