Solid fats are liquefied by heat, and freed from the tissue that contains them in animal fats like suet.
When a fat begins to smoke with heat, a chemical change is taking place. If intense heat is continued, all the hydrogen and oxygen are driven off and pure carbon remains. When the fat is "brown," giving the flavor we like, a part of the oxygen and hydrogen have been driven off. The "boiling" of fat in a kettle is ordinarily due to the boiling of the water contained in the fat.