3. Concrete operations stage. This stage lasts from about 6 or 7 years until about 11 years of age. During this stage, children acquire new thinking skills to work with actual objects and events. They are able to imagine undoing an action, and they can focus on more than one feature of a problem. Children also begin to understand that there are points of view different from their own. This new awareness helps them master the principle of conservation. A child in the concrete operations stage will understand that six apples are always six apples, regardless of how they are grouped or spaced, and that the amount of clay does not change as a lump is molded into different shapes. This ability is not present in the preoperational stage. However, instead of thinking a problem through, children in this stage tend to rely on trial-and-error strategies.