This study describes the improvement in 20 sixth grades students' reasoning abilities in the context of structured or semi-structured inquiries conducted during an after-school science club. The findings shed light on the improvement in student reasoning and on the specific areas of student difficulties. Overall reasoning skills showed more or less continuous improvement; whereas, the warrants changed in a non-linear pattern--like waves--with crests of improvement and troughs of setbacks. The study also suggests that writing played an important role in the process of student learning. (Contains 3 tables and 2 figures.)