This case study describes Senior high school students' argumentation and problem- solving process during a scientific inquiry about ”What causes the black drop effect”. Its purpose is to provide a reference for teachers whom guide scientific inquiry. The inquiry team consisted of two teachers and two tenth-grader boys, but one boy withdrew one year later. During the process of problem solving, teachers used ”hill climbing” and brainstorming as strategies. The ”why” question was converted into ”what” and ”how” questions. In this case, the question ”why the black drop effect happen” was converted into ”what variables affect the black drop effect” and ”how do the variables affect the black drop effect”. Then, do the inquiry activities through induction, and argument by hypothetic-deduction to find out the explanation for the causes of the black drop effect. This case study shows that the conversion of ”why” into ”what” and ”how” is a key to successful problem-solving.