  The characteristics of extensive air showers are sensitive to the details of
hadronic interactions at energies and in kinematic regions beyond those tested
by human-made accelerators. Uncertainties on extrapolations of the hadronic
interaction models in these regions hamper the interpretation of the ultra high
energy cosmic ray data in terms of primary mass composition. We report on how
the Pierre Auger Observatory is able to constrain the hadronic interaction
models by measuring the muon content and muon production depth of air showers
and also by measuring the proton-air cross section for particle production at a
center-of-mass energy per nucleon of 57 TeV.
