You all have some cash. Perhaps not on you right now, because payments are increasingly becoming digital—indeed, for me, it’s been a while since I stopped carrying a wallet. But you’re all well familiar with banknotes, obvi- ously. Now, imagine money that, if you didn’t spend it by a certain date, would self-destruct—like the glasses with the secret instructions in the opening credits of Mission Impossible II. And I’m not just metaphorically talking about “self- destructing” in the sense of being eroded by inflation. No, I’m talking about currency that completely disappears if unspent: you can’t spend it any more after a certain date.