Based on the predictions of a computational model, we test preschoolers' ability to reason about stream location effects: reasoning that interventions that occur on a common part of a causal process should be more likely to affect multiple relations, than interventions which occur on independent parts of a causal process. In two experiments, we show that 3- and 4-year-olds both show a stream location effect. Children show this effect for both familiar and unfamiliar interventions.