The goal of this paper is to explore the use of dynamic geometry environments (DGE) as a type of computer programming language to learn geometry. Using classroom projects created by upper elementary students in Web Sketchpad (more specifically, closely following the construction process of a Leonardo da Vinci or mirror-writing machine by a team of two boys), we describe and analyze the computational thinking practices of debugging/decomposition and abstraction, which prominently came to the fore during the construction process, and show how these practices – along with the students’ drive for aesthetics, and the well-designed tools– propelled one another and ultimately supported evolution of spatial reasoning and the learning of reflectional symmetry.