The document contains a passage about different types of psychological research: experimental, differential, and developmental. It notes that Piaget's work on human development established developmental psychology as a unique category of study. While his theories were influential, his methodology has been criticized for a lack of standardized experiments and defined data. The passage also describes two types of boomerangs - returning boomerangs which fly in a circular path and return, and non-returning boomerangs which can be used as weapons due to their heavier weight and design.