  A fascinating class of patterns, often encountered in nature as meandering
cracks on rocks, dried-out fields and tectonic plates is produced by the
fracture of solids. Here we report the observation and modeling of an unusual
type of patterns consisting of spiral cracks within fragments of a thin layer
of drying precipitate. We find that this symmetry-breaking cracking mode arises
naturally not from twisting forces, but from a propagating stress front induced
by the fold-up of the fragments.
