According to Jean Piaget's theory of cognitive development, children achieve the ability to demonstrate the conservation of quantities (such as volume, mass, or number) despite changes in their physical shape or appearance during which stage?

According to Jean Piaget, mathematical knowledge is primarily acquired through:

How does Piaget's theory explain children's process of learning mathematical concepts?

A Class 2 teacher notices that when she pours water from a short, wide glass into a tall, thin one, her students insist that the taller glass holds more water. Based on Piaget's theory, which stage of cognitive development are these students most likely in, and what concept are they struggling with?