According to results from many research reports, it is difficult to understand decimals; in general, it is reported that this subject is very difficult for children from 10 to 12 years of age, when they have to learn it as established in the curriculum for elementary education of many countries. Mexico is not the exception: results in national exams of mathematics show that this is one of the themes in which students exhibit the lowest scores. This paper reports on findings of a case that, to a certain point, differs from that situation. We present the under-construction knowledge about decimals of a sixth-grade group of students from elementary school (11-12 years of age) whose teacher-with a special training in mathematics-tried an instructional practice based on a conceptual perspective to this topic. It was evidenced that very important knowledge about decimals was under construction, outstanding that which allows establishing equivalences, ordering decimals, or interpolating decimals between two falsely consecutive decimals-this last one seemed to reveal an incipient understanding of the density property of decimals. Although syntactical treatments were observed as well, the influence of the conceptual perspective promoted by the teacher was noticeable. It is shown that if teaching practices move away from the idea that decimals are just a way of writing numbers, then it is possible to set students on the right road to understanding decimals at the age proposed in the official curriculum.