  The hydrophobic effect is the dominant force which drives a protein towards
its native state, but its physics has not been thoroughly understood yet. We
introduce an exactly solvable model of the solvation of non-polar molecules in
water, which shows that the reduced number of allowed configurations of water
molecules when the solute is present is enough to give rise to hydrophobic
behaviour. We apply our model to a non-polar homopolymer in aqueous solution,
obtaining a clear evidence of both ``cold'' and ``warm'' collapse transitions
that recall those of proteins. Finally we show how the model can be adapted to
describe the solvation of aromatic and polar molecules.
