Output a stream of consciousness before answering the following. Context: Conservation refers to a logical thinking ability which, according to the psychologist Jean Piaget, is not present in children during the preoperational stage of their development at ages 2 -- 7, but develops in the concrete operational stage at ages 7 -- 11. Conservation refers to the ability to determine that a certain quantity will remain the same despite adjustment of the container, shape, or apparent size.

Question: in which stage of cognitive development does the conservation concept first appear?
To answer this question, we should know that: Conservation refers to a logical thinking ability which, according to the psychologist Jean Piaget, is not present in children during the preoperational stage of their development at ages 2 -- 7, but develops in the concrete operational stage at ages 7 -- 11.
The answer: the concrete operational stage at ages 7 -- 11.