As children watched, Dr. Vallance poured the same amount of juice into two identical glasses. Then she poured the juice from one of those glasses into a taller, narrower glass. When asked which glass now has more juice, children in the concrete operational stage say that?

That both glasses have the same amount of juice

Explanation:

Jean Piaget developed a theory that focuses on cognitive development, according to him, we go through different stages in our cognitive development since the moment we are born, this stages are qualitatively different (from one to the other).

Piaget's four stages of cognitive development are: sensorimotor stage, preoperational stage, concrete operational stage and formal operational stage.

Concrete operational stage takes place between 7 and 11 years, and one of the most important milestones in this stage is the concept of "conservation": this term refers to the ability that allows a person to determine that a certain quantity will remain the same despite adjustment of the container, shape, or apparent size.

Piaget actually made this experiment with children in the preoperational stage and children in the concrete operational stage where he would pour the same amount of liquid into identical glasses. Then he would pour the juice from one of the glasses into another taller, narrower glass. Younger children would say that the taller glass contained more liquid because it's taller (focusing only on one dimension), while the kids on the concrete operational stage would say that the liquid remained the same and that he just changed the container.

In the example, Dr. Vallance did the same experiment Piaget did, since concrete operational stage children have developed the concept of conservation, they will say that the amount of juice is the same in both glasses.