A surprise bout of inflation can trick producers into working harder, but only briefly
Robert Lucas imagined an economy of separated 'islands,' each producer seeing only their own local price, not the overall price level. When the money supply unexpectedly increases, a producer can't tell whether a higher price reflects real extra demand for their good or plain economy-wide inflation, so they rationally produce more just in case. That confusion, Lucas showed, is why unanticipated monetary policy can briefly move real output, while a policy everyone sees coming in advance can't.