Transactions are defined using a Forth-like scripting language.[3]:ch. 5 Transactions consist of one or more inputs and one or more outputs. When a user sends bitcoins, the user designates each address and the amount of bitcoin being sent to that address in an output. To prevent double spending, each input must refer to a previous unspent output in the blockchain.[67] The use of multiple inputs corresponds to the use of multiple coins in a cash transaction. Since transactions can have multiple outputs, users can send bitcoins to multiple recipients in one transaction. As in a cash transaction, the sum of inputs (coins used to pay) can exceed the intended sum of payments. In such a case, an additional output is used, returning the change back to the payer.[67] Any input satoshis not accounted for in the transaction outputs become the transaction fee.[67]
While there is certainly the possibility of making short-term profits in Bitcoin, many market participants are viewing an investment in Bitcoin as a long-term play. If the cryptocurrency were to eventually become a favored form of global payment and remittance, there is no telling just how high prices could go. Some have even suggested that the price of Bitcoin could hit $50,000 in 2018 and eventually $1 million.
In January 2015 Coinbase raised 75 million USD as part of a Series C funding round, smashing the previous record for a bitcoin company.[109] Less than one year after the collapse of Mt. Gox, United Kingdom-based exchange Bitstamp announced that their exchange would be taken offline while they investigate a hack which resulted in about 19,000 bitcoins (equivalent to roughly US$5 million at that time) being stolen from their hot wallet.[110] The exchange remained offline for several days amid speculation that customers had lost their funds. Bitstamp resumed trading on 9 January after increasing security measures and assuring customers that their account balances would not be impacted.[111]
If fewer people begin to accept Bitcoin as a currency, these digital units may lose value and could become worthless. There is already plenty of competition, and though Bitcoin has a huge lead over the other 100-odd digital currencies that have sprung up, thanks to its brand recognition and venture capital money, a technological break-through in the form of a better virtual coin is always a threat.
Though perhaps not useful for the proverbial “cup of coffee” today, bitcoin is regularly used to move hundreds of millions of dollars across borders, often much more quickly and cheaply than settling such transactions through the conventional financial system. Earlier this month, for instance, a crypto user sent $194 million worth of bitcoin for just $0.10.
On 19 June 2011, a security breach of the Mt. Gox bitcoin exchange caused the nominal price of a bitcoin to fraudulently drop to one cent on the Mt. Gox exchange, after a hacker used credentials from a Mt. Gox auditor's compromised computer illegally to transfer a large number of bitcoins to himself. They used the exchange's software to sell them all nominally, creating a massive "ask" order at any price. Within minutes, the price reverted to its correct user-traded value.[186][187][188][189][190][191] Accounts with the equivalent of more than US$8,750,000 were affected.[188]

There are no physical bitcoins, only balances kept on a public ledger in the cloud, that – along with all Bitcoin transactions – is verified by a massive amount of computing power. Bitcoins are not issued or backed by any banks or governments, nor are individual bitcoins valuable as a commodity. Despite its not being legal tender, Bitcoin charts high on popularity, and has triggered the launch of other virtual currencies collectively referred to as Altcoins.
“So if you look at a $100 billion market cap today, now last week it might have been more like 200, so it’s actually a buying opportunity, we think that there’s a potential appreciation of 30 to 40 times because you look at the gold market today, it’s a $7 trillion market. And so a lot of people are starting to se that, they recognize the store of value properties.”

The U.S. Commodity Futures Trading Commission has issued four "Customer Advisories" for bitcoin and related investments.[14] A July 2018 warning emphasized that trading in any cryptocurrency is often speculative, and there is a risk of theft from hacking, and fraud.[168] A February 2018 advisory warned against investing an IRA fund into virtual currencies.[169] A December 2017 advisory warned that virtual currencies are risky because:

After a certain amount of transactions have been verified by a miner, they will receive newly minted bitcoins for their work and thus new bitcoins will be added into circulation, while the number of bitcoins in circulations are now in the multi-millions range, the maximum amount of  bitcoins that can ever be created is capped at 21 million. The creation rate is automatically halved every few years as more bitcoins are added into circulation, whilst this system is modeled after gold, mining difficulty is always increasing and makes finding new bitcoins more rare as the number of available bitcoins reaches the 21 million cap.

More recently, at a congressional hearing on Capitol Hill, global economist and New York University professor Nouriel Roubini said, "Crypto is the mother or father of all scams and bubbles." He called out "swindlers" who tapped into investors' fear of missing out and took them for a ride with "crappy assets that went into a bust and crash — in a matter of months — like you have not seen in any history of financial bubbles."

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