How would you explain the concept of fiat currency to someone who has only known the gold standard?

I am aware that the silver and gold standard used to be nigh universal in the world. I also know from reading about post and pre war histories that a return of the gold standard was seen as a prerequisite for stabilization and served to "prevent excess spending" (their words not mine).

And then we have today's situation, where currency isn't backed by anything, and the value seems to be backed by faith in government and its influence. What would be the first questions that gold standard users would have for the current system?