Novel causal systems pose a problem of variable choice: How can a reasoner decide which variable is causally relevant? Which variable in the system should a learner manipulate to try to produce a desired, yet unfamiliar, casual outcome? In much causal reasoning research, participants learn how a particular set of preselected variables produce a particular effect. Here, we investigate 3- to 5-year-olds' ability to select the relevant variable for intervention in a novel causal system. Results demonstrate that even young children can learn which variable is causally relevant from sparse evidence. In particular, children infer that variables that are "difference-making" in one causal system will also be relevant to other, novel, causal problems. If manipulating a causal variable in a particular way leads to one effect, children assume that other manipulations of that variable will lead to other novel effects.