Two studies of the innovative semantic distinctions and innovative uses before the acquisition of conventional number classifiers by young Japanese children (aged 5-7 years) are discussed. The findings suggest that lexical acquisition is an intricate process which often requires more than simple mappings of forms onto categories, and that children's search for convention is very persistent. Children often conform to convention by modifying and refining their semantic categories on the basis of input data even though they have already mapped the items onto "good" conceptual categories, and they seem to continue their search for convention long after they have attained good productive control over the items, and when there is no communicative need to persist. (MSE)