  The weak cosmic censorship conjecture asserts that spacetime singularities
that arise in gravitational collapse are always hidden inside of black holes,
invisible to distant observers. This conjecture, put forward by Penrose more
than four decades ago, is widely believed to be one of the basic principles of
nature. However, a complete proof of this hypothesis is still lacking and the
validity of the conjecture has therefore remained one of the most important
open questions in general relativity. In this study we analyze a gedanken
experiment which is designed to challenge cosmic censorship by trying to
overcharge a Reissner-Nordstr\"om black hole: a charged shell is lowered {\it
adiabatically} into the charged black hole. The mass-energy delivered to the
black hole can be red-shifted by letting the dropping point of the shell
approach the black-hole horizon. On the other hand, the electric charge of the
shell is not red-shifted by the gravitational field of the black hole. It
therefore seems, at first sight, that the charged shell is not hindered from
entering the black hole, overcharging it and removing its horizon. However, in
the present study we prove that the exposure of a naked singularity to distant
observers is actually excluded due to the formation of a new (and {\it larger})
horizon around the original black hole. Moreover, we shall prove that this new
horizon is already formed {\it before} the charged shell crosses the original
black-hole horizon. This result, which seems to have been previously
overlooked, guarantees the validity of the weak cosmic censorship conjecture in
this type of gedanken experiments.
