  While professional integrated programming environments support developers
with advanced debugging functionality, block-based programming environments for
young learners often provide no support for debugging at all, thus inhibiting
debugging and preventing debugging education. In this paper we introduce
NuzzleBug, an extension of the popular block-based programming environment
Scratch that provides the missing debugging support. NuzzleBug allows
controlling the executions of Scratch programs with classical debugging
functionality such as stepping and breakpoints, and it is an omniscient
debugger that also allows reverse stepping. To support learners in deriving
hypotheses that guide debugging, NuzzleBug is an interrogative debugger that
enables to ask questions about executions and provides answers explaining the
behavior in question. In order to evaluate NuzzleBug, we survey the opinions of
teachers, and study the effects on learners in terms of debugging effectiveness
and efficiency. We find that teachers consider NuzzleBug to be useful, and
children can use it to debug faulty programs effectively. However, systematic
debugging requires dedicated training, and even when NuzzleBug can provide
correct answers learners may require further help to comprehend faults and
necessary fixes, thus calling for further research on improving debugging
techniques and the information they provide.
