Children sometimes make "scale errors," attempting to interact with tiny object replicas as though they were full size. Here, we demonstrate that instrumental tools provide special insight into the origins of scale errors and, moreover, into the broader nature of children's purpose-guided reasoning and behavior with objects. In Study 1, 1.5- to 3.5-year-olds made frequent scale errors with tools in a free-play session. Study 2 utilized a novel forced-choice method, representing a stronger test by handing 2-year-olds a feasible alternative for goal achievement, but children continued to make scale errors. Study 3 confirmed that errors were not based in perceptual immaturity. Results are explained using a framework of teleofunctional (purpose-based) reasoning as a powerful and early developing influence on children's actions. (Contains 3 tables and 2 figures.)