The United States crossed an extraordinary threshold this month: $40 trillion in gross federal debt. That includes $32.3t owed to the public and another $7.8t that the government owes to its own accounts, such as the Social Security trust funds. Numbers this large tend to become abstractions, but there is a simple explanation for how we accumulated $40t: politicians routinely authorize more services than the government collects enough taxes to pay for. Until the incentives change, this number will continue rising and we as a society will face the consequences [...]