Each interview began by providing the students
with three sets of different colored clay balls, which
could be used during the interviews at any time. The use
of clay balls during interviews was implemented in
response to students' initial difficulties with using a
drawing to represent their ideas about an atom during a
set of pilot interviews. The clay balls could be easily
paired together or reshaped and thus provided a way for
students to create a three-dimensional physical model to
represent their mental picture of an atom (Wefelmeier,
1937). We hoped that students would find the clay balls
helpful when trying to describe what makes an atomradioactive and to illustrate more subtle details of the
decay process at the atomic level