Three minute explainer of the Brexit paradox

An agreement was made long ago to maintain an open border between Northern Ireland (part of the UK) and the independent country of Ireland (part of the EU). The problem is, when the UK leaves the EU, what happens to the open border between Northern Ireland and Ireland? How could Northern Ireland be part of the EU and the UK at the same time? Will it have to become part of Ireland to maintain the open border? CGP Grey examines this predicament, concluding that there are no answers that will make everyone happy.