It stretches!
Mozzarella is a stretched curd cheese (Pasta Filata). It is best produced from very fresh milk in which the natural or added dairy bacteria convert the sweet lactose in the milk to lactic acid. When the acid level reaches a specific point and the curd is held in hot water, it changes from the normal curd structure to a cheese stretching out into long elastic strands that are worked into the smaller forms and shapes.