Procedure and materials Children participated in four sessions: (a) brief warm-up play, (b) natural play with mothers (MN),(c) experimental play with mothers (ME), and (d) experimental play with the experimenter (EE). Thewarm-up play lasted three minutes, and each of the remaining sessions (MN, ME, EE) lasted 10 min.To avoid inducing mothers to pretend, the MN condition always preceded the ME condition, whereasthe EE condition either preceded or followed the MN–ME sequence.