It may look hard to frame a house, but it used to be a lot harder. Kevin O’Connor explores the incredible evolution of timber to stick framing. The 2×4 helped America spread out across the plains, but it reduced Chicago to ashes along the way. History professor Dr. Dennis Cremin traces the flames, and This Old House’s Jeff Sweenor helps snuff them out. Great innovation paired with massive tragedy set the stage for modern-day homebuilding.