Several studies have emphasized the predominant influence of students' conceptions in the learning process in physics.  But, more often than not, these conceptions are somehow considered in a negative way, as "errors."  In this article we describe another approach, where students' conceptions were explicitly taken as an active basis for scientific reasoning in real class situations (level:  sixth and fourth French grades - American Grades 6 and 8, respectively - approximately 12 and 14 years old). The subject matter taught was basic electricity.  Using clinical interviews, preinstruction conceptions were first established.  The article describes the first five sessions of teaching, and shows how these conceptions were actually used (or not used) by the students to explain electricity phenomena, and how the conceptions subsequently changed, increasing in internal consistency without moving closer to scientific conceptions.  To overcome this situation, teachers provided an analogical explanation. Students proved able to use the physical analogy as well as to specify its limits.