Final answer:
Preoperational children, typically aged 2-7, lack an understanding of conservation, which is the concept that the properties of objects remain the same despite changes in form. This understanding develops in the concrete operational stage, from about 7-11 years old, when children can think more logically.
Step-by-step explanation:
Preoperational children do not have an understanding of conservation because they are in a stage of cognitive development where they cannot yet perform mental operations. Conservation is the concept that certain properties of an object, like mass, volume, or number, remain the same despite changes in the forms or arrangements of the objects.
For example, in the context of Piaget's task, if a child is shown water being poured from a tall, thin glass into a short, fat glass, they might believe there is more water because the water appears to take up more space in the short glass, indicating they do not understand conservation.