"Incidental teaching" denotes a process whereby language skills of labelling and describing are learned in naturally occurring adult-child interactions. In the present study, 15-min daily samples of the spontaneous speech of 11 children were recorded during free play over eight months of preschool. After incidental teaching of compound sentences, increases in unprompted use of compound sentences were seen for all the children, first directed to teachers, and then to children, in accordance with who attended to the children's requests for play materials. The incidental teaching procedure also stimulated spontaneous variety in speech, and appears to have general applicability to child learning settings.