Einstein showed with General Relativity that the universe doesn’t necessarily need to be a “flat” geometry. But when you hear the word “flat,” you might think of a sort of pancake shape. Is that what Professor Guth means about the shape of our universe? What does it mean to be “flat,” geometrically? How can we know for sure if our universe is “flat”? How would we know if it were some other non-Euclidean geometry?