Those portions of the animal that do the work are tougher than those that do not. Two main components of the connective tissue that Jim refers to are collagen and elastin. The amount of these two components varies, depending on the part of the animal, sex, and age. Elastin is yellow and can be easily seen and cut out. Elastin will not become tender from cooking; it shrinks and becomes tougher. Collagen is white, small fibers, which are not visible to the naked eye, unless a lot of it forms sheets and strands. Collagen will break down, becoming soft and tender, when properly cooked. There are two cooking methods that can be used to make this happen - keep it at an elevated temperature for an extended period of time, or cook it at very elevated temperatures, in a moist environment, such as a pressure cooker. In the dry heat environment of a smoker, the cooking temperature must be kept high enough to break down the collagen and render out the fat. At the same time, it must be kept low enough that it will not evaporate excessive moisture. Remember, water turns to steam and evaporates at 212?F. As the fat melts and renders out, it will replace some of the water, but not all. Thus, ~ 225? has been found to be an ideal cooking temperature for tough cuts of meat, like brisket and shoulder. Cooking it to an internal temperature of 195?F to 205?F is as close to the boiling point of water that one can get, without losing excessive moisture, and allows the increased temperature to render out the fat and break down the collagen. Once thought about and analyzed, the science part of cooking makes sense. Enjoy!