Billy refuses to drink his orange juice from the 1/2 full glass that his mother gives to him. he wants her to pour the juice into his favorite cup and watches his mother fill it to the brim. billy likes his cup better because he gets more juice in it. with what cognitive concept in piaget's theory is billy having trouble?

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.