This paper presents the process of preparing a guide for teaching and learning about the study of “living beings”, its implementation in the classroom, the analysis of the construction of the meaning of “living being” by students, and the assessment of their learning. The practice of Experimental Reflective Science Teaching underlay the elaboration of this guide, which was implemented in three 1st grade classes (n=64). The analysis of the process of teaching and learning occurred in class is centered on the class diaries, which were produced as a result of participant observation in the context of the classroom, and it seeks to: a) identify the students' spontaneous ideas about the meaning of “living being”; b) to promote the (re)construction of a new meaning of “living being”; c) to assess, in contrast to a control group (n=64), the specific effect of this teaching practice in student learning. The results have shown that most students in the experimental group, with an average age of 6.5 years, have not been able to develop a meaning of “living being”, as part of a complex conceptual framework However, through the intervention, a significant part of the experimental group developed a meaning of “living being” which is well beyond that proposed by the 4th Piagetian stage of development, scheduled for the age of 11/12 years, and according to which life is reserved for animals, or for animals and plants.