Next, the smith pulls the katana from the fire and plunges it into a trough of water in a rapid cool-down process called "quenching." Because the sword's back edge and inner core contain very little carbon, they can contract more freely than the high-carbon steel at the front edge of the blade. The difference in both the degree and speed of contraction between the two forms of tamahagane causes the sword to bend, creating the distinctive curve. This is a tricky stage, in which as many as one in three swords is lost.