Final answer:
The first substage of Piaget's pre-operational thought is the symbolic function substage, where children can mentally represent absent objects using symbols like words and images.
Step-by-step explanation:
According to Piaget, the first substage of pre-operational thought is the symbolic function substage, in which the child gains the ability to mentally represent an object that is not present. During this stage, children begin to use symbols such as words and images to form mental representations of objects that are not in their immediate environment. For example, a child may have a stuffed animal that they associate with comfort, and even when the toy is not physically present, the child can still think about it and feel comforted.