The concrete operational stage is a key phase in Jean Piaget's theory of cognitive development, typically occurring between ages 7 and 11. During this stage, children begin to think logically about concrete events and understand the concept of conservation, which means they can recognize that quantity doesn't change even when its shape does. Children in this stage also develop skills in classification and seriation, allowing them to organize objects based on shared characteristics or arrange them in a logical order. However, their thinking is still tied to tangible objects and experiences, making abstract reasoning more challenging.