The study explores whether the naive concepts of "solid" and "liquid" persist in adolescence. Accuracy of responses and reaction times where measured while 41 ninth graders classified different solids (rigid, non-rigid and powders) and different liquids (runny, dense) into solid or liquid. The results show that these naive conceptions affect adolescences' classifications in terms of both accuracy and reaction time. The rate of correct classifications of non-rigid solids and powders was significantly lower than of rigid solids. Lower rate of success was also found for classification of dense liquids compared with runny liquids. In addition, the reaction time results of correct classifications for non-rigid solids and powders were longer than those for rigid solids and, likewise, reaction times for dense liquids were longer than for runny ones. These results suggest that reasoning processes associated with correct classification of objects that are not consistent with the naive conceptions are more demanding.