Final answer:
In Piaget's concrete operational stage, children achieve the understanding of conservation, recognizing that altering an object's appearance does not change its quantity, and they comprehend the principle of reversibility.
Step-by-step explanation:
An accomplishment in Piaget's concrete operational stage is a child's ability to understand the concept of conservation. Around the ages of 7 to 11 years old, children begin to grasp that the mass, volume, or number of objects remains the same despite changes in the form or appearance of the objects. This is highlighted in the example with Keiko and Kenny where Keiko understands that despite Kenny's pizza being cut into more pieces, the amount of pizza they each have is still the same because nothing was added or removed. This stage also involves the principle of reversibility, which suggests that children in this stage can understand that objects can be changed and then returned back to their original form or condition.
In Piaget's concrete operational stage, children develop the ability to think logically about real, concrete events. They have a firm grasp on the use of numbers and can perform mathematical operations. Additionally, children in this stage master the concept of conservation, which is the idea that even if something changes shape, its mass, volume, and number stay the same.