  This paper describes a cost effective and safe device to perform realistic
experiments about the physics of radioactivity in classrooms. It can be used to
study both $\alpha-$ and $\beta-$radioactivity as well as $\gamma-$emitters and
shows an extremely realistic behaviour. The device, in the form of a
Geiger-M\"uller tube, was tested during a public lecture and it deceived many
people, even among physicists and teachers.
