What makes a brisket different from a flank steak?  Like, how come one kind of meat is perfectly cooked after a quick sear, but other other needs hours of slow-roasting to be tender? Well, it mostly comes down to the connective tissue called collagen. Meat comes from muscle, which is just strands of specialized proteins bound together by collagen. And cooking breaks collage down into gelatin, which gives meat that melt-in-the-mouth texture. As collagen ages, those fibers get bigger and more connected, especially in the muscles that get more work. The more crosslinks of collagen there are the longer it will take for the meat to get truly tender. So a rib-eye, which comes from the rib area, can be perfectly tender with just a light sear. But a brisket, which comes from the chest area and supports a lot of the cow's wight, takes hours of cooking to tenderize.