  The critical behaviour of several spin models can be simply described as
percolation of some suitably defined clusters, or droplets: the onset of the
geometrical transition coincides with the critical point and the percolation
exponents are equal to the thermal exponents. It is still unknown whether,
given a model, one can define at all the droplets. In the cases where this is
possible, the droplet definition depends in general on the specific model at
study and can be quite involved. We propose here a simple general definition
for the droplets: they are clusters obtained by joining nearest-neighbour spins
of the same sign with some bond probability p_B, which is the minimal
probability that still allows the existence of a percolating cluster at the
critical temperature T_c. By means of lattice Monte Carlo simulations we find
that this definition indeed satisfies the conditions required for the droplets,
for many classical spin models, discrete and continuous, both in two and in
three dimensions. In particular, our prescription allows to describe exactly
the confinement-deconfinement transition of SU(N) gauge theories as Polyakov
loop percolation.
