In 1773 Coulomb(1) recognized that the static properties of granular system
s can be discussed in terms of the frictional properties between different
layers(2), leading to his relationship between the angle of repose of a gra
nular pile (theta(0)) and the coefficient of static friction mu(s): tan the
ta(0), = mu(s). Two centuries later, solid friction and granular media stil
l present many puzzles. One such is that the coefficient of static friction
depends on the time during which the solids remain in contact before the m
easurement. Here ive show that this ageing effect is manifested too in the
angle of repose of granular media and originates from capillary condensatio
n of water vapour between the packed particles, leading to the formation of
water bridges. By assuming that the kinetics of this process are governed
by the thermally activated nucleation of bridges, we can reproduce both the
time- and humidity-dependence of the ageing behaviour. Our results also cl
arify the kinetics of adsorption in porous media more generally.