Piaget's theory of cognitive development

One example of an experiment for testing conservation is the water level task. An experimenter will have two glasses that are the same size, fill them to the same level with liquid, and make sure the child understands that both of the glasses have the same amount of water in them. Then, the experimenter will pour the liquid from one of the small glasses into a tall, thin glass. The experimenter will then ask the child if the taller glass has more liquid, less liquid, or the same amount of liquid. The child will then give his answer.

I always thought this test was "children think taller and thinner = more". But actually it's way different, this test is "children think taller and thinner = more, but they think it so hard that they think the quantity of liquid literally expands to fill the space."