This meeting discusses an on-going research which concerns children's difficulty with the semantic interpretation of additive particles. Previous studies report that Japanese-learning children have difficulty interpreting sentences that contain the additive particle 'mo', often accepting them under context adults would likely reject. However, we have yet to figure out why children make such error. The present research seeks to address this issue by investigating whether or not better experimental design could improve children's poor performance with 'mo'. We will conduct the same experiment on English-learning children to test their interpretation of 'also'.