At the peak about $18 trillion in bonds had negative yields, but actually making individuals pay to leave money in the bank has always been tricky. They could just stuff cash into mattresses. To get around this, a 19th-century economist suggested having a stamp duty on money every year in order for it to remain legal tender. This would kill two birds with one stone since it would be pretty awkward for a criminal like Walter White in "Breaking Bad" to dig up the $80 million he buried in the desert, much less to explain it. More recently, economist Greg Mankiw published a student's idea in 2009 that random serial numbers on bills could be declared void through a lottery, making hanging on to cash risky