Emergency powers are designed for when plans need to change, and fast, by allowing the president to override certain Constitutional provisions in a time of crisis. So how has the national emergency gone from a rarity to a tool that presidents use dozens of times while in office? We look at what a president can and cannot do during a state of emergency, and how Congress has tried to put checks on that power, with help from Kim Lane Scheppele, author of Law a the Time of Emergency.