Three experiments investigated preschoolers’ ability to infer that numbers can be causally efficacious. Preschoolers observed that one of two quantities of objects activated a machine (i.e., a container holding 2 blocks activated a machine while a container holding 3 did not). Children were asked to determine whether novel containers with either 2 or 3 objects would activate the machine, and then construct their own container of objects that would do so. Four-year-olds, but not 3-year-olds, were above chance at both tasks given a contrast between the numbers ‘2’ and ‘3’ (Study 1), but not as good when the contrast was between the numbers ‘4’ and ‘6’ (Study 2) The effect of age on understanding ‘2’ was mediated when children’s numerical knowledge was considered (Study 3). These results are interpreted in terms of children’s causal reasoning and hypothesis-formation abilities, but also their developing knowledge of numbers.