Analogy is among the most widely studied pedagogical tools in science education, and every major theoretical framework governing its use, from Glynn's Teaching With Analogies model to Harrison and Coll's FAR Guide to Niebert et al.'s embodied analogy framework, treats the presence of shortcomings as a necessary and inescapable feature of analogical instruction. This paper challenges that assumption as a general principle. The Identical Scale Principle (ISP) is introduced, which proposes that an analogy has zero shortcomings if and only if two conditions are jointly satisfied: the source domain and the target concept are governed by the same physical law operating at different scales, and the student possesses direct sensory experience of the source domain. Drawing on physical law invariance, embodied cognition theory, and a systematic analysis of five confirmed examples, this paper demonstrates that the ISP defines a previously untheorized class of analogies that are not merely low in shortcomings but are structurally incapable of producing analogical distortion. The gap in the existing literature is documented, the theoretical rationale is developed, five exemplary cases are analyzed in depth, implications for teaching practice are drawn, and an empirical research agenda is proposed. The principle has significant implications for analogy selection, curriculum design, and the pedagogical frameworks that govern analogical instruction.

