Studying computer science (CS) in elementary schools has gained become popular in recent years. However, students at such a young age encounter difficulties when first engaging with CS. Robotics has been proposed as a medium for teaching CS to young students, because it reifies concepts in a tangible object, and because of the excitement of working with robots. We asked: What CS concepts can elementary-school students learn from the participation in a robotics-based CS course? We used two theoretical frameworks to explain possible difficulties in learning: the Jourdain effect, and constructs vs. plans. A taxonomy of six levels was created to characterize levels of learning. The levels were measured using four questionnaires that were based on the taxonomy. In addition, field observations of the lessons were recorded. The population consisted of 118 second-grade students (ages 7-8). Lessons on CS concepts using Thymio educational robot and its graphical software development environment were taught during normal school hours, not in a voluntary extracurricular activity. The syllabus was based on existing learning materials that were adapted for the young age of the students. The analysis showed that the students were very engaged with the robotics activities. They did learn basic CS concepts, although they found it difficult to create and run their own programs. We concluded that the Jourdain effect was not demonstrated because the students understood concepts and constructs of CS; however, they were unable to plan and construct their own programs from the basic constructs.