  Two interleaved phonebooks are nearly impossible to separate by pulling their
spines. The very slight force exerted by the outer sheets of the assembly is
amplified as the exponential of the square of the number of sheets, leading to
a highly-resistant system even with a small number of sheets. We present a
systematic and detailed study of the influences of the normal external force
and the geometrical parameters of the booklets on the assembly strength. We
conclude that the paper-paper adhesion force between the two outer sheets, on
the order of a few mN, is the one that is amplified by the two interleaved
system. The two-phonebooks experiment that has attracted the attention of
students and non-scientific public all around the world as an outstanding
demonstration of the strength of friction appears to be as well a spectacular
macroscopic manifestation of the microscopic friction-adhesion coupling.
