What happens during a greenstick fracture?

A greenstick fracture is characterized by a unique form of break that resembles how a green twig bends and breaks. In this type of fracture, the bone does not break completely through; instead, it bends on one side and creates a fracture on the opposite side. This partial break is why the correct answer highlights that the bone is both partially bent and partially broken.

In contrast, the other options describe different types of fractures. A complete break where the bone is fully separated into two pieces is indicative of a complete fracture, while a caving in or crushing of the bone suggests a different form of damage known as a compression fracture or a comminuted fracture, where the bone is shattered into multiple pieces. Thus, the distinction of the greenstick fracture lies in its partially broken and bent nature, accurately represented by the correct answer.