Box plots are frequently used, but are often misinterpreted by students. Especially the area of the box in box plots is often misinterpreted as representing number or proportion of observations, while it actually represents their density. In a first study, reaction time evidence was used to test whether heuristic reasoning underlies this misinterpretation. Moreover, it was tested whether this heuristic reasoning was caused by the saliency of the area of the box. In a second study, the effect of an intervention aimed at improving students' interpretation of the box plot on their heuristic reasoning about box plots was investigated. The results suggest that this misinterpretation of box plots is indeed due to heuristic reasoning and is very difficult to overcome. (Contains 6 figures and 2 tables.)