Two experiments investigated whether a simple-to-embedded-rules account can explain the change in causal reasoning in children between 3 and 4 years of age. A marble-and-ramp apparatus that operated in 2 distinct configurations of straight and across was used throughout. In Experiment 1, 3-year-olds were able to predict the path of the marble when there was only 1 input hole (a simple if-then rules task), whereas only 4-year-olds could solve the 2-input version (an embedded or conjoint conditional if-then rules task). Experiment 2 found the same 3- to 4-year age difference when children chose where to insert the marble in the 2-input version, indicating that the same rules may underlie causal action as well as causal prediction for the more complicated task. The results of the 2 experiments are discussed in relation to previous findings on causal reasoning, children's theory of mind, and a theory of cognitive complexity in the preschool period.