Sources and corrections
True, or it does not go in.
The ten lessons make 108 factual claims. Every one is listed below with its source. All were last checked on 4 October 2026.
How we check facts
- 01Every claim has a sourceEach lesson has a fact sheet listing every claim it makes and the page that supports it. Claims without a source are left out.
- 02Primary sources firstWe prefer the original: regulators, official documentation, protocol specifications and published data. A claim we have only seen reported second-hand stays out until we have read the original.
- 03Figures carry a dateNumbers that change, such as market sizes and fees, are shown with an "as of" date. We recheck them and update the date.
- 04Corrections are listedWhen we find a mistake, we fix it and record the correction on the lesson and on this page.
Corrections
None yet.
Sources by lesson
Lesson 01What is a blockchain?15 claims · 10 sources
- Bitcoin fingerprints its blocks with SHA-256, applied twice.Bitcoin developer reference: block chain · Bitcoin Project
- A blockchain is a shared record of payments: new payments are grouped into blocks.Bitcoin developer guide: block chain · Bitcoin Project
- Each block gets a hash, a short fixed-length code calculated from its contents.Bitcoin developer reference: block chain · Bitcoin Project
- A hash works like a fingerprint: it is computationally infeasible to work back to the input or find two inputs with the same hash.Glossary: cryptographic hash function · NIST Computer Security Resource Center
- Each new block stores the fingerprint of the block before it.Bitcoin developer reference: block chain · Bitcoin Project
- Follow those links back and you reach the very first block.Bitcoin genesis block (block 0) · mempool.space
- Alter an old record and its block gets a different fingerprint; the next block still holds the old one, so the link breaks.Bitcoin developer guide: block chain · Bitcoin Project
- Thousands of computers around the world each keep their own copy (12,105 Ethereum nodes found on 4 October 2026).Ethereum node tracker · Etherscan
- Those computers check every new block and accept it only if its payments are valid.Bitcoin: A Peer-to-Peer Electronic Cash System · Satoshi Nakamoto (Satoshi Nakamoto Institute copy)
- A copy that has been changed simply disagrees with everyone else's, and the network ignores it.Bitcoin developer guide: block chain · Bitcoin Project
- Hard to change is not impossible: rewriting history is computationally infeasible, not impossible.Glossary: cryptographic hash function · NIST Computer Security Resource Center
- Bitcoin's history was rewritten after a software fault in 2010.Value overflow incident · Bitcoin Wiki
- Bitcoin's history was rewritten after a software fault in 2013.BIP-50: March 2013 chain fork post-mortem · Bitcoin Improvement Proposals
- Ethereum Classic was attacked in 2020 by an attacker who controlled most of its computing power.Attacker stole 807K ETC in Ethereum Classic 51% attack · Bitquery
- Smaller blockchains have repeatedly been attacked by miners who controlled most of the computing power.51% attacks · MIT Digital Currency Initiative
Lesson 02Keys and wallets15 claims · 8 sources
- A self-custody wallet creates a secret number called a private key.Bitcoin developer guide: transactions · Bitcoin Project
- Only that key can sign a payment from your address.Bitcoin developer guide: transactions · Bitcoin Project
- From the key, the wallet works out an address.Ethereum accounts · ethereum.org
- The address cannot be turned back into the key.Ethereum accounts · ethereum.org
- Your coins are not inside the wallet: the wallet is a window onto the blockchain and holds the keys.Ethereum wallets · ethereum.org
- The blockchain records which address owns the coins.Bitcoin developer guide: transactions · Bitcoin Project
- Wallets often show a recovery phrase to write down.Ethereum wallets · ethereum.org
- A recovery phrase is 12 to 24 words, taken from a fixed list of 2,048.BIP-39: Mnemonic code for generating deterministic keys · Bitcoin Improvement Proposals
- Anyone with those words can rebuild the key.BIP-32: Hierarchical deterministic wallets · Bitcoin Improvement Proposals
- If you lose both the wallet and its recovery phrase, nobody can recover the coins.Some things you need to know · bitcoin.org
- The recovery phrase is the only way to recover a self-custody wallet.Ethereum wallets · ethereum.org
- Share the phrase and the coins can be taken: scammers use it to recreate the wallet.Ongoing phishing campaigns · Ledger
- Wallet makers such as Ledger say they will never ask for your recovery phrase; scammers posing as support staff do.Ongoing phishing campaigns · Ledger
- On an exchange, the exchange holds the keys for you.Ethereum wallets · ethereum.org
- Anyone can see the balance and history of an address you share.Protect your privacy · bitcoin.org
Lesson 03Sending a payment13 claims · 8 sources
- Your wallet builds the payment and signs it with your key.Ethereum transactions · ethereum.org
- The signature proves you approved it without revealing the key.Bitcoin developer guide: transactions · Bitcoin Project
- The signed payment is sent to the network and waits with other pending payments in a pool, often called the mempool.Ethereum transactions · ethereum.org
- Payments offering higher fees are usually picked first.Gas and fees · ethereum.org
- A block that includes the payment is its first confirmation, and every block added after it is one more.Some things you need to know · bitcoin.org
- For large Bitcoin payments, the usual guidance is to wait for six confirmations.Some things you need to know · bitcoin.org
- Fees depend on demand for space in blocks.Bitcoin developer guide: transactions · Bitcoin Project
- Space in each block is limited.BIP-141: Segregated Witness (block weight limit) · Bitcoin Improvement Proposals
- On 4 October 2026 an average Ethereum transaction cost about $0.07, and on layer 2 networks about $0.002.Layer 2 · ethereum.org
- Layer 2 networks batch transactions and post them to Ethereum.Optimistic rollups · ethereum.org
- A confirmed payment cannot be reversed.Some things you need to know · bitcoin.org
- Coins sent to a wrong address are usually gone for good: only the receiver can send them back.Some things you need to know · bitcoin.org
- Scammers plant look-alike addresses in your history.Address poisoning scams · Chainalysis
Lesson 04Bitcoin9 claims · 7 sources
- Bitcoin miners repeat a calculation until one of them finds a valid fingerprint for the next block.Bitcoin developer guide: mining · Bitcoin Project
- The winner is paid in newly created bitcoin, plus the fees of the payments inside the block.Bitcoin: A Peer-to-Peer Electronic Cash System · Satoshi Nakamoto (Satoshi Nakamoto Institute copy)
- The new-bitcoin reward halves every 210,000 blocks, about every four years: 50, 25, 12.5, 6.25 and now 3.125 bitcoin per block.Controlled supply · Bitcoin Wiki
- The reward has been 3.125 bitcoin per block since 20 April 2024 (block 840,000).Bitcoin block 840,000 · mempool.space
- Because the reward keeps halving, the total can never reach 21 million (the ceiling is 20,999,999.9769).Controlled supply · Bitcoin Wiki
- About 20.09 million bitcoin had been created by 4 October 2026, 95.7% of the cap (computed from the block height and the reward schedule).Current block height · mempool.space
- Under a million bitcoin is still to come, released ever more slowly, with the last expected around 2140.Controlled supply · Bitcoin Wiki
- Bitcoin's price has repeatedly fallen by half or more: about 83% from late 2013 to early 2015, 77% in 2018, 76% in 2022 and 53% from October 2025 to July 2026 (computed).Historical bitcoin price (US dollars) · mempool.space
- The FCA warns that anyone buying crypto should be prepared to lose all their money.Crypto basics · Financial Conduct Authority (InvestSmart)
Lesson 05Stablecoins10 claims · 8 sources
- Only approved customers, mostly companies, deal with the issuer directly; everyone else buys tokens on an exchange.Primary and secondary markets for stablecoins · Federal Reserve (FEDS Notes)
- The issuer takes in dollars and creates the same number of tokens, and pays back a dollar for each token returned.Circle Mint · Circle
- Reserves are mostly short-term US government debt and similar assets.USDC transparency and reserves · Circle
- About $316 billion of stablecoins existed on 4 October 2026; USDT and USDC made up about four fifths.Stablecoins data · DefiLlama
- Nearly all stablecoins are pegged to the US dollar.Annual Economic Report 2025, chapter III: the next-generation monetary and financial system · Bank for International Settlements
- Stablecoins serve mainly as a way into and out of crypto trading.Annual Economic Report 2025, chapter III: the next-generation monetary and financial system · Bank for International Settlements
- Retail-sized payments are a tiny share of stablecoin transfers.Making sense of stablecoins · Visa
- In March 2023, $3.3 billion of USDC's reserves was stuck at the failed Silicon Valley Bank and its price fell below 90 cents.Primary and secondary markets for stablecoins · Federal Reserve (FEDS Notes)
- USDC recovered once US authorities protected the bank's depositors.Joint statement by Treasury, Federal Reserve and FDIC, 12 March 2023 · Federal Reserve
- TerraUSD, which had no reserve of dollars, collapsed in May 2022.Crypto-enabled fraudster sentenced for orchestrating $40 billion fraud · US Department of Justice (SDNY)
Lesson 06Exchanges8 claims · 6 sources
- A centralised exchange keeps an order book: buyers' prices on one side, sellers' prices on the other.Matching engine · Coinbase Exchange documentation
- When a buyer's price meets a seller's, the exchange matches them.Matching engine · Coinbase Exchange documentation
- The trade happens at the price of the offer that was already waiting in the book.Matching engine · Coinbase Exchange documentation
- Coins you keep on an exchange are under its keys: you are trusting the exchange with custody of them.Ethereum wallets · ethereum.org
- Exchanges can be hacked, fail or freeze access to funds.Securing your wallet · bitcoin.org
- In August 2026 about four fifths of tracked crypto spot trading happened on centralised exchanges (decentralised exchanges had 18.7%).Exchange Review, August 2026 · CoinDesk Data
- FTX stopped withdrawals in November 2022 and collapsed (paused 8 November, bankrupt 11 November).SEC v. Samuel Bankman-Fried, complaint, 13 December 2022 · US Securities and Exchange Commission
- Hackers stole about $1.5 billion from Bybit in February 2025.North Korea responsible for $1.5 billion Bybit hack · FBI Internet Crime Complaint Center
Lesson 07Swaps and pools9 claims · 4 sources
- On a decentralised exchange, a pool holds two tokens supplied by other people.Uniswap v2 Core whitepaper · Uniswap
- You swap against the pool, and a program on the blockchain does the trade.Uniswap v2 Core whitepaper · Uniswap
- The pool keeps its two balances, multiplied together, from falling; before fees the product stays the same.Uniswap v2 Core whitepaper · Uniswap
- This constant-product rule is how Uniswap's original (v2) pools set the price.Uniswap v2 Core whitepaper · Uniswap
- Put 1,000 USDC with a 0.30% fee into a pool of 10 ETH and 20,000 USDC and you get 0.4748 ETH, not 0.5 (arithmetic from the whitepaper's rule).Uniswap v2 Core whitepaper · Uniswap
- The bigger the swap compared with the pool, the worse the price.Uniswap v2 Core whitepaper · Uniswap
- Others can see your swap before it is final and trade around it (a sandwich attack).Maximal Extractable Value: implications for crypto markets · European Securities and Markets Authority
- Sandwich attacks cost traders on Ethereum about $60 million in the year to October 2025.Exclusive data from EigenPhi reveals that sandwich attacks on Ethereum have waned · Cointelegraph Research
- A slippage limit caps how bad a price you accept.Uniswap v2 pricing · Uniswap documentation
Lesson 08Staking10 claims · 6 sources
- Ethereum is secured by validators, each of which locks up a deposit called a stake.Proof-of-stake · ethereum.org
- Each validator locks up at least 32 ETH.Pectra · ethereum.org
- Validators propose and vote on new blocks.Proof-of-stake · ethereum.org
- Once validators holding at least two thirds of the staked ether have voted for it in two rounds of checkpoints, a block is final, about 15 minutes after it was added.Proof-of-stake · ethereum.org
- Validators that do their job earned about 2.6% a year as of 4 October 2026.Ethereum validator queue · validatorqueue.com
- About 43.7 million ETH was staked on 4 October 2026, a little over a third of all ether.Ethereum validator queue · validatorqueue.com
- A validator that signs two conflicting blocks is slashed: part of its deposit is destroyed and it is removed.Ethereum staking · ethereum.org
- On 4 October 2026 the queue to leave took about 14 days.Ethereum validator queue · validatorqueue.com
- Rewards change over time: they fall as more ether is staked.Proof-of-stake rewards and penalties · ethereum.org
- Staking through a provider such as Lido means its node operators run the validators and it keeps 10% of the rewards.Lido documentation · Lido
Lesson 09Lending and borrowing9 claims · 5 sources
- On a lending protocol such as Aave, you deposit crypto as collateral and borrow less than it is worth.Aave V3 overview · Aave documentation
- Borrowers are anonymous, so loans are secured by collateral rather than a credit check.DeFi lending: intermediation without information? · Bank for International Settlements (Bulletin 57)
- The health factor is the collateral counted at its liquidation threshold (80% here) divided by the debt: $10,000 of ether against $6,000 gives 1.33.Liquidations · Aave Help Center
- Below a health factor of 1, the loan can be liquidated.Liquidations · Aave Help Center
- If ether falls 26%, the collateral is worth $7,400 and the health factor drops to about 0.99, below 1 (arithmetic from the formula).Liquidations · Aave Help Center
- Anyone can then repay part of the debt and take some of the collateral at a discount.Liquidations · Aave Help Center
- About $31 billion was borrowed across lending protocols on 4 October 2026 (computed).Protocols data · DefiLlama
- In April 2026, an attacker used tokens released by a bridge exploit as collateral to borrow about $193 million from Aave.rsETH incident report, 20 April 2026 · Aave governance forum
- Aave's own code was not broken, but some lenders could not withdraw for a time.rsETH incident report, 20 April 2026 · Aave governance forum
Lesson 10Tokenised assets10 claims · 5 sources
- A fund manager buys ordinary assets, such as short-term US government debt.USYC · Circle
- The fund issues tokens on a blockchain, each recording a share in the fund.USYC · Circle
- Circle's USYC is an example: each token is a share in a fund holding short-term US government debt.USYC · Circle
- BlackRock's BUIDL is another tokenised US Treasury fund.Tokenized Treasuries · rwa.xyz
- The tokens circulate on public blockchains, but only between wallets the issuer has approved.The rise of tokenised money market funds · Bank for International Settlements (Bulletin 115)
- Tokenised US government debt was worth about $14.8 billion on 4 October 2026.Tokenized Treasuries · rwa.xyz
- On the same tracker, stablecoins were worth about $295 billion that day.Tokenized real-world assets overview · rwa.xyz
- In the UK, the first digital gilt is due by the end of March 2027.Update on the digital gilt instrument (DIGIT) pilot issuance · HM Treasury
- Tokenised funds carry the risks of the fund behind them.The rise of tokenised money market funds · Bank for International Settlements (Bulletin 115)
- Many tokenised funds are not open to the public: USYC, for example, is only for non-US persons, with a $100,000 minimum.USYC · Circle