Stress can actually exist in a material in the absence of an applied load. This is known as residual stress, and it can be used as a way to toughen materials, such as in the fabrication of the Japanese Katana sword. Conversely, undesired residual stresses can encourage crack growth and lead to fracture, such as with the collapse of the Silver Bridge of West Virginia in 1967. Perhaps the most striking example of residual stress is related to the rapid cooling of molten glass, known as “Prince Rupert’s Drop”: