In many cases, cryptocurrencies cannot be converted to real currencies; it is only possible to convert them to other cryptocurrencies, or to use them to buy things. Some cryptocurrencies can be converted to real currencies: They usually have a high volatility, and using them carries a high risk. They are also a target for so-called Pump-and-Dump-Attacks. They act like a big distributed economic system: as they are not issued or controlled by central banks, their value is difficult to influence: For this reason, they cannot really take the place of a stable currency.