Studies have shown that learning mathematics through programming can be complex and that the programming itself might even hamper students' learning. However, few studies have focused on the role of the teacher and the teacher-student interaction that aims to support students' learning when using programming. The present study examines a didactic design in which a teacher uses well-prepared questions and suggestions that focus on students' reasoning to solve mathematical problems using Scratch, a block-based programming environment. Forty students, 10-11 years old, solved a geometry problem using Scratch supported by their teacher. The students' screen activities and teacher-student interactions were recorded. The results indicate that well-prepared general and task-specific questions targeting students' creative reasoning can help to overcome some of the complexities of learning mathematics while programming. The study furthermore shows that when students have sufficient programming skills, Scratch could even have the potential to support their reasoning and at the same time guide the teacher to give timely feedback.