This study examines the proposal in Cognitive Grammar (Langacker 1987, Talmy 2000 that grammatical classes are iconically motivated. The discussion follows a case-study to test this hypothesis. Using found data, we examine the productivity of a range of grammatical classes across Dutch, English, and German. The study bases its analysis on the lexical concept of precipitation. The perceptual and universal nature of such a concept should be a best-case scenario for iconic motivation of grammatical classes. However, despite this, the test-case produces mixed results. Although the hypothesis is not disproved, we reveal how it cannot, alone, explain the vagaries of lexical -class grammaticality.