For years, tommy and his younger brother billy argued every night at dinner over who was to get the bigger glass, even though both glasses held the same amount of liquid. one glass was tall and thin, and the other was short and wide, and each boy always wanted to have the "bigger" glass for himself. one evening, to the delight of his parents, tommy said to his mother, "let billy have the taller glass. i know they both hold the same amount." according to piaget, this event is indicative of which stage of cognitive development?