Billy is having trouble in the concept of conservation, which is part of the concrete operational stage of cognitive development.
Trouble in conservation means that Billy is unable to understand that a change in the form of an object does not necessarily mean that the quantity of it has changed. In this instance, Billy has not acquired the ability to understand the 1/2 full glass of orange juice does not increase in quantity when it's poured in his favorite (smaller glass) and reaches its brim.
