Cryptocurrency
As a cryptocurrency holder, you cannot cash out for what you put in, since the miners and cryptocurrency service providers "take a cut".
- this makes them a costly form of currency
The rising price of cryptocurrency is based solely on demand for an asset with no functional value
- see also Greater Fool Theory
Mining
the correct answer to these puzzles can be easily verified by anyone on the network, though being the first to find the answer requires an enormous amount of processing power — and thus electricity
The point of having an incredibly complex math problem for miners to solve is so that adding new blocks becomes so difficult, that meddling with the blockchain is prohibitively expensive.