Osmosis and the movement of water is a common theme in biology courses. Students first learn the concept in General Biology (Bio181/Bio156), then students have to apply it to human physiology in Anatomy and Physiology (Bio 201/202). Students have a difficult time understanding this concept as evidenced by only 39% correctly answering a question about osmosis in the kidney on the unit exam. To address this issue, I utilized a whiteboarding activity to have students critically evaluate how changing osmolarity in the kidney medulla affects water movement from the collecting duct. I assessed their understanding by scoring a question on the unit exam. I found that 54% of students correctly answered the question. Thus, using the whiteboarding activity had a positive impact on student understanding of osmosis in kidney physiology.