By coins or scrap metal only. Or seeds, because at some point when you find out that your 401K “invested” is only dots on someones computer screen.
Move on folks, nothing to see here, Your government and banks have been doing this for a long, long time.
Actually it's been since the early 1600's and we don't complain because we want it this way.
When we put say $1,000 in the bank, sure we could pay the bank to back our deposit with $1,000 in tens and twenties but we don't because we'd rather have them back the $1k with something like $2k worth of platinum and let the bank pay us interest.