  The study is about a basic shadow experiment, which was enriched with a
simulation to understand the reasoning of participants when we use a trick. Two
light sources create an umbra and penumbra behind the objects. With this
experiment, we asked what would happen when the penumbras interact. Most of the
participants predicted the correct solution, that there should be an umbra.
Some of the participants choose wrong alternative, and explained in terms of
the structure of penumbra.
