Physical and cognitive development is rapid during early childhood. According to Piaget, children begin to represent the world through language, images, and drawing during the preoperational stage. Vygotsky sees dialogue as important for language development, and believes language and thought initially develop independently then merge. All mental functions have social origins. Development occurs through zones of proximal development and scaffolding with more skilled individuals. Debate continues around curriculum approaches, with advocates for both child-centered and direct instruction models.