Cognitive development in children?

Oct 2010
Jean Piaget was a Swiss psychologist who developed a very influential theory of cognitive development, or how our thinking changes as we grow. Make up a family with two children at two different stages of Piaget's theory. How old are these children, what stage are they in, and what might we see in their behavior that indicates their level of cognitive development?
May 2011
she has developed 4 stages in addition to that she expriment the conservation task with the A cup B cup at the same messure with a child observation she pured the B cup to C cup while its thin and longer when you ask a child which cup has more water they would most likely to say the C while they all the same amount.
this experiment i find very intersting, because the child sees the proccess and still cant define it correctly.
why is that? is it because they look for the outside which is the cup? or they just dont think with common sens?