  A model based on the existence of two different competing local structures in
water is described. It is shown that it can explain the transition between
fragile and strong behavior that supercooled water has around 220 K. The high
temperature behavior is similar to that observed in standard fragile
glass-formers. The strong behavior at low temperatures is associated with the
existence of a remanent configurational entropy coming from the possibility of
locally choosing between the two possible environments.
