When fresh milk is left to become sour, the casein aggregates. If souring occurs at not too low a temperature and without stirring or shaking of the milk, a gel is formed. When keeping the gelled or clotted milk, some whey separation (syneresis) generally occurs. Syneresis can be enhanced by heating and stirring; the mass then separates into curd and whey. By allowing more of the whey to leak from the curd, e.g., by hanging the curd in a cloth, a primitive fresh cheese is obtained (baker's cheese, quarg, or simply "curds").