  Electrification by rubbing different materials is a well known phenomenon
with a history that begun more than five centuries BC. However, simple
experiments can lead to contradictory or inconsistent results and the history
of this phenomena is plagued with non-intuitive results. For example,
triboelectric charging by rubbing identical materials is possible. In this work
we want to highlight some historical aspects of triboelectricity that could
enrich the discussion of electrostatics in an undergraduate physics course. We
will focus on the effect of strain on the triboelectric properties of a sample,
which we think is not well known to physics teachers. We will show that it is
possible to obtain robust polarities by rubbing identical rubber balloons of
different radii and we will also show that this charging method can be very
useful in introductory physics courses.
