Renal clearance is one of the more difficult concepts for students of physiology to learn. We hypothesized that this difficulty is rooted in a student's misunderstanding of virtual volume. This was tested by having students select from several drawings the one they thought plasma would look like after a certain volume of it has been cleared of sodium by the kidneys. About half the participating students selected plasma pictured as having a certain volume of it devoid of sodium molecules. That is, their misconception of clearance seemed to be due to a lack of understanding about virtual volume, a deficiency which is reinforced by the classic definition of clearance. To address this misconception, a demonstration was devised in which a beaker of concentrated colored water was used to represent plasma before renal clearance, a beaker of the same concentrated colored water in which the top third had been replaced by clear mineral oil was used to represent what the definition of clearance said would happen to plasma after a third of it had been cleared of sodium, and a beaker of dilute colored water was used to represent what really happens to plasma when a certain volume of it is cleared of a solute. Incorporating this demonstration into discussions of renal clearance helped students to understand this concept, as evidenced by improved scores on related questions.