In psychology, and according to Piaget, conservation refers to a stage of logical understanding and mental development that is arrived at between ages 7 and 11 ( the concrete operational stage in Piaget's stages of cognitive development).
This is when a child's mental processing starts to comprehend that a quantity of matter, such as water or sand, remains the same no matter what shape of container contains the matter. This can be demonstrated by putting the same quantity of sand or water in two or more different shaped containers and asking the child which one of the containers contains more of the measured substances. So you have 8 ounces of water in two different sized cups: a small wide one and a tall skinny one. Before a child understands conservation they will say there is more water in the taller glass because it appears taller. After children understand the law of conservation they will know that there is the same amount of water in both glasses and they just appear to be different sizes.