[horofox]

Tell them that it's just a currency, but unlike a normal currency where the government has control, nobody has control.

Then there's a big sheet of paper where the transactions with their amounts are written and shared with everybody, that way everybody knows how much each of them has. So when they receive money, they know that whoever sent them has money.

Then just raise up the simple problem of you copying that last transaction and transferring money to somebody else before everybody knows that you did the first. You can just say that there is a system that waits until that other transaction is processed and be done with it. It makes no sense to actually explain in more depth even to a savvy user...

even if you do explain, it will be like when they try to explain poverty in the third world to Americans... they can even see the picture, but they won't understand because they've never lived it.

In the end, all you do is to think that you managed to explain something but the other party didn't really understand you