Oh, stuff like that has to do with general chemical properties. Off the top of my head, I think baking powder is essentially baking soda with something added (cream of tartar? sulfate? Can't recall) and baking soda is sodium bicarbonate, a moderate base that reacts with most weak acids (such as vinegar - acetic acid). Acid/base reactions generally release carbon dioxide and water, the carbon dioxide and the water form tiny pockets in the flour/etc, then are released during the baking process by the heat. It essentially leaves it looking like pumice on the microscopic scale, scaling down the density of the baked thingie and also usually making it moist due to the water being released as steam but the steam being caught in the cookie.<br/><br/>Or whatever.<br/><br/>Egg yolks are protein, and scrambling eggs involves on the molecular level essentially breaking the way the proteins fold with themselves and with other proteins. See, on the molecular level, proteins prefer to ball up and fold in and around each other kind of like a stray thread that you roll between your hands and you end up with a chaotic little thread-ball. Proteins do this best in the environment where they naturally exist, so (for example) human-body proteins will prefer to be at a pH of 7.4 and a temperature of 98.6, and changing the pH or the temperature makes them very unhappy. Same with egg proteins. And when proteins are unhappy, they unfold and end up kind of laying like uncooked spaghetti with each other. That's why they get destroyed with the introduction of heat suddenly - I'm not sure specifically how 'tempering' an egg works, though.<br/><br/>An emulsion (emulsification is the verb for how you make an emulsion, at least in chem) is a mixture of usually liquid fat and something watery. The technical definition is two things that normally would not mix, mixed. (E.g. butter and water-based broth) I'm not sure what you mean by why doesn't it break, though. >.><br/><br/>The questions you're asking are pretty much general molecular properties applied to baking. Good stuff.