This study deals with the school instruction of the concept of weight. The historical review reveals the major steps in changing weight definition reflecting the epistemological changes in physics. The latest change drawing on the operation of "weighing" has been not widely copied into physics education. We compared the older instruction based on the "gravitational" definition of weight with the newer one based on the "operational" definition. The experimental teaching was applied in two versions, simpler and extended. The study examined the impact of this instruction on the middle school students in regular teaching environment. The experiment involved three groups (N = 486) of 14-year-old students (ninth grade). The assessment drew on a written questionnaire and personal interviews. The elicited schemes of conceptual knowledge allowed to evaluate the impact on students' pertinent knowledge. The advantage of the new teaching manifested itself in the significant decrease of the well-known misconceptions such as "space causes weightlessness," "weight is an unchanged property of the body considered," and "heavier objects fall faster". The twofold advantage--epistemological and conceptual--of the operational definition of weight supports the correspondent curricular changes of its adoption.