So, you're in a restaurant and you're ready to try and guess the tip size so that the waiter serves you as quickly as possible, but it turns out that this restaurant has its own currency for tips: dinner must be paid in one currency, and tips in another! And you have no options - this is how all the restaurants in this world are arranged.
- What a blessing that this is a fictional story and fictional restaurants - you say.
– We agree! There are no such restaurants! we say.
Unfortunately, this is exactly how everything works in existing cryptocurrencies: you have a cryptocurrency, but in order to use it, you also need «gas» - the internal cryptocurrency of the network to pay the commission. This problem has been resolved in SDN.