Aqueous foams, like other macroscopically divided materials, display
intriguing rheological properties. The bubble-scale structure allows for
the existence of frozen stresses within the material which can not
spontaneously relax by thermal activation. Upon shearing, the system
undergoes a series of plastic events which irreversibly modify this
internal stress pattern. Reversely, the internal state of the material
controls to a large extent its mechanical response to shear.