Students in Dutch primary schools spend quite some time working on mathematics. The average lessons however are limited to relatively short interactive introductions of new content; the rest of the time pupils spend on paper and pencil work. There is little time for mathematical reasoning and problem solving that inspires both students and teachers. In 2004 the Freudenthal Instutute of Utrecht University started an annual event (the 'Grote Rekendag', the Big Mathematics Day) to promote inquiry learning in the mathematics lessons of primary school. In 2016 we organized this event with the theme "Let's have a look behind the code", a theme inspired by the activities from 'CS unplugged' and by other educational ideas and the upcoming interest in coding and programming for young children. Using interviews (pupils age 9-12 and teachers) and a questionnaire we investigated what pupils and teachers liked about the theme and the activities and what they think they learned from these. Results show that teachers and pupils liked the activities. Teachers indicated that their pupils learned about coding and procedures, and less about how a computer works.