In the concrete operational stage children’s thought becomes more logical. Logical thought uses reason when thinking. Children are able to understand changes between things that are in front of them. They still have a hard time with things that they have to imagine. Children in the concrete operational stage can place items in different arrangements. They understand that things can be placed into more than one group at a time. Children in the concrete operational stage can also put things in order of qualities like length and weight. The understanding of conservation happens in the concrete operational stage. Children who understand conservation know that changing a things shape or container does not change how much is there. For example, in conservation of liquid children know that the amount of liquid does not change if it is put in a different glass.