Many students do not understand that fluid pressure is the cause of the buoyant force. This work designed a process called question-answer teaching method to help improve this understanding. Sets of questions were developed which can be catagorized into three types: concept-guiding questions, comprehension-checking questions, and concept-applying questions. The major process of the method is that the instructor uses sets of the designed questions in class to encourage students and challenge their understanding. By expressing and discussing their ideas with peers and with the instructor, their misconceptions could be corrected, and the concepts could be understood by themselves. To prove if this teaching method is effective, a study was conducted with two groups of students: a control group (N = 88) and an experimental group (N = 12). A multiple-choice conceptual test was provided as pre-test and post-test. It was found that the results of the two groups are significantly different (p-value = 0.01). The normalized gain of the traditional group is 0.08 while that of the experimental group is much higher, at 0.63. Moreover, the students in the experimental group showed better understanding. They could conclude that the buoyant force results from the pressure difference in fluids.