U.S. Treasury Secretary Janet Yellen warned on Monday that the U.S. could default on its debt as early as June 1. Over recent years, the U.S. has raised its debt limit multiple times, yet climbing federal debt continues to hit the debt ceiling. What are the reasons behind this? What kind of impacts would the U.S. debt-limit crisis bring onto itself and the global economy? Here's what you need to know about the U.S. debt-ceiling crisis.