We have been concerned so far with developments in affect expression and saying words. In this chapter, we turn to how the children played with the toys in the playroom as a window on developments in cognition. We will see in particular how developments in object play, saying words, and emotional expression were integrated – both in the children's development over the course of the year and a half in which we studied them and in the moment-to-moment events in the stream of spontaneous activity in which we observed them.

Karin Lifter and I used several sorts of data from the children's play for comparison with developments in language. The primary data came from play in which a child did something with two objects in relation to each other. The child either took two objects apart – for example, taking a bead off the string – or constructed a thematic relation between objects – like putting a doll in the truck. The categories of play activity derived from these data, and the sequence of their emergence and achievement are the central results of the study.

Secondary data were also collected to provide a developmental context for these results and to help us understand them. One source of secondary data was the children's spontaneous finding behavior: how they located objects in the playroom to use for constructing relationships between them.