 Jun asks, what can you put on the Bitcoin blockchain? Andreas, how many transactions can fit in a block with and without SegWit? In addition, what kind of other data can you include in a block? Thanks. Jun, great question. I can't give you an exact number, because it depends on the nature of transactions. Transactions can vary greatly in their size. The most basic transaction, which is a single input, single output, paid-to-public-key hash transaction, if I remember correctly, is 266 bytes. The average transaction has one input and two outputs. It's a bit bigger than that, because it has changed. If it uses a more complex construct, perhaps paid-to-script hash, it's going to be a bit longer. If it uses a multi-sig and is paying out of a multi-sig, it's going to have some redeem scripts. That's a bit longer, too, and so on and so forth. The transaction size can vary. On average, a one-megabyte block can fit about 1,800 transactions, I guess, maybe up to 2,000 transactions. Now that we have segregated witness on the Bitcoin main chain, that can expand to, I would say, about 4,500 transactions in a single block. You can look at the statistics, I haven't looked at them recently, to see exactly how many transactions. Certainly, we've seen an almost doubling of the capacity of the block in order to fit more transactions. I think the factor was 1.7 last time I looked. The second part of the question is also interesting. What kind of other data can you include in a block? Keep in mind, blockchains are the world's most expensive and inefficient form of data storage. They are so structured intentionally because decentralization requires us to make some sacrifices in terms of efficiency. That means you can't store a lot of data, and when you try to store a lot of data, it gets very expensive. However, one of the interesting things you can do with a blockchain is that instead of storing the data itself, you can store a hash or fingerprint of the data, and then store the data in a secondary network, such as IPFS, which is the interplanetary file system, or any other form of storage. You could even store your data on a Google Drive if you wanted to use a centralized service. With that, what you can do is have the data stored outside, but you can take a fingerprint or hash of your data and embed that in the blockchain. What good does that do? What it does is it gives you the ability to prove that a certain piece of data existed on or before a certain time. That is called proof of existence. That is being a useful service for legal purposes. You can do notarization for a variety of uses. In fact, there is a protocol that allows you to compress many such hashes, hundreds of thousands, millions of them, into a single transaction that timestamps every block. That protocol is called open timestamps, which allows you to timestamp all kinds of data into the Bitcoin blockchain. Similar protocols exist for other blockchains that can be used to timestamp data. Marcus asks, what exactly is meant with metadata? Are metadata visible and useful? Yes, it refers to something external to the system in which it is stored. In Bitcoin, all the data that is about Bitcoin is data. Any data that is not about Bitcoin but about something else that is stored inside Bitcoin is metadata. That is one of the ways to look at it. Of course, the term is probably a bit more flexible and vague than that. How could I upload a hash of a document in an opera-turned-field myself? Is there a special command on Bitcoin Core which allows me to do so? I don't believe there is. There are a number of other applications. Probably the best... There are a couple of good ones. One is called Proof of Existence. I believe it is at proofofexistence.com, which allows you to upload a hash of a document and put it in an opera-turned-field for a small fee. Another one is a more developed protocol called Open Timestamps, which is a project by Peter Todd. It scales the ability to do proof of existence for billions of documents by aggregating all of this information in merkle trees. Putting only the root of the tree in each opera-turned-transaction while storing most of the metadata off the chain on Open Timestamps servers. It is a good compromise solution. Open Timestamps is what you are looking for there. Marcus asks, in the same manner, could I upload any text I want on the Bitcoin blockchain, as long as it is less than 80 bytes? Yes, you can. Finally, Marcus asks, could Ethereum be used more efficiently for storing hashes of documents instead of the Bitcoin blockchain? And why? Yes, Ethereum could be used more efficiently for storing hashes of documents instead of the Bitcoin blockchain. The primary reason is that Ethereum will allow you to store a lot more data, and as a result, has bigger scaling issues than the Bitcoin blockchain. On Bitcoin, you can store 80 bytes in each opera-turned-transaction, and it will cost you a transaction fee to include that in the transaction. With Ethereum and a smart contract, you can store quite a lot of data, and you still have to pay a transaction fee, but there is no 80-byte limit. It is really a matter of metering that with gas. In fact, there is a particular type of contract called a deed, or non-fungible token, which is basically storing hashes of things. These can be deeds like real estate titles, or crypto-kitty hashes, or various other things that are non-fungible, unique items, and storing those hashes inside a tokenized contract that allows you to transfer these tokens from person to person, or from owner to owner. That is called ERC721. It is rather interesting. Tracy asks, can a block with hash data, such as an academic certificate hash stored in an opera-turned-transaction, also have regular, mineable transactions and therefore be treated just like any other block? Tracy, yes. In fact, the transaction that has the opera-turned-transaction in it can also have other things like regular payments. Only one of the outputs needs to be an opera-turned-transaction. The other outputs in that transaction could be payments, so you can embed it in a transaction that does other things, as well as the opera-turned-transaction. Of course, that can be among many other transactions, and the block is just like any other block.