 Bob asks, are miners spending block rewards and mining fees? Am I correct in understanding that miners cannot spend the block reward until after 100 blocks have been mined? If so, is there a waiting period before miners can spend the mining fees, or can they spend them earlier? Bob, you are correct. There is a coin-based transaction, which contains both the block subsidy, which is the 12.5 new bitcoins at the current rate. The fees, all in one transaction, the coin-based transaction, cannot be spent for 100 blocks after it is mined. This is to protect against miners gaming the system with deep reorganizations. In order to do that, they can't actually spend the rewards until 100 blocks have passed, at which point you have a lot more proof-of-work piled on top of that chain. It is much more difficult to reorganize it, because you don't want miners spending the rewards, and then have the chain reorganized. That would cause havoc in Bitcoin. Both the subsidy and the fees, which are in the same coin-based transaction, if you look at a block today, and you look at the first transaction, which is called a coin-based transaction, you'll see that it's not 12.5 Bitcoin, it's more. The amount above 12.5 Bitcoin is the sum of all of the fees remaining in the block. You can check the math, if you want, sit down and add up all of the fees in all of the transactions, and see if it's the same as the difference between the coin-based transaction amount and the 12.5 Bitcoin that is the block subsidy at present. You'll see that it is exactly the same or less. You can have a coin-based transaction that earns less, but not more. So it's one transaction, fees and block subsidy are in the same transaction, and it's the coin-based transaction that cannot be spent for 100 blocks. That makes attacks against the chain of reorganizations enormously risky for miners, because they may never, ever get that fee. If all the miners try to go back to the Genesis block, how much energy do they need to consume in order to change it? The simple answer is the same amount of energy that was used to mine everything in the first place. That's not actually correct. The reason it's not actually correct is because the equipment that was used to mine the early blocks was much less efficient than the equipment that is used today. Instead of taking the same amount of energy, it's going to be a smaller percentage of energy than the amount of energy that was used in the first place. While it will be done a lot faster, because there is much more hashing power available today, the faster hashing won't be that much more efficient in terms of energy used, because faster hashing means more energy is being used. In fact, the amount of energy that would need to be consumed in order to change the blockchain all the way back to the Genesis block is a big chunk of all of the energy that was already used to mine it in the first place. That's one of the necessary security mechanisms that makes 51% attacks, difficult, expensive and unsustainable, and also that makes immutability strong in a proof-of-work system. You mentioned something about immutability of the whole chain being impossible to counterfeit or change because of the sheer volume of work with proof-of-work. Can we maintain proof-of-work with the current energy demands that it needs? Or do we need to move to proof-of-stake and sacrifice this immutability in some sense? Can we sustain proof-of-work? Or do we move to proof-of-stake? If you try to move to proof-of-stake, only a few will follow you. So these types of fundamental principled decisions no longer result in A or B. They result in A and B through the mechanism of a fork, which means there will always be a proof-of-work network as long as I'm running it. I have a choice as one individual to continue to run it. If there's two of us, better. If there's two million of us, great. If it's just me in my basement, I'm the last die-hard supporter of proof-of-work. I've been in that position before. I've been called weird. It's okay by me. The interesting thing about crypto is that you don't have to make that choice. In fact, in the absence of overwhelming support to move to something new, proof-of-work strongly preserves the status quo by forcing a fork, which means we're not going to leave proof-of-work. It's not going to happen at all. It's never going to happen. It might become very expensive to run. In that case, people will turn off their equivalent, the difficulty will go down, and then it will no longer be too expensive to run. It's a dynamic algorithm. How much energy are we using right now? We're using exactly the right amount of energy. How do I know? Because the market has told me so. There are two ways to make decisions in this world. One is to aggregate all of the information available to the market through the power of price. And the other one is to elect someone to make the decision for you. We already have systems where you can elect someone to make the decision for you. This is a market-based system. And as long as it exists as a market-based system, it will aggregate all of the profit and loss decisions made by all of the participating miners, and say, how much is the right amount of proof-of-work? The current amount of proof-of-work is the right amount of proof-of-work, by definition, because the market said so. Great question to end on. Thank you so much for coming. Thank you.