sentences:  The custom persisted until the late 19th century, and inspired the "men of iron" statues that were used for propaganda and fund-raising in Germany and Austria in World War I, particularly the tree set up in Freiburg.The most likely explanation for the medieval nails is the ancient custom of hammering nails into crosses, trees and even rocks for protection or in gratitude for healing, that is as a votive offering, similar to throwing coins into a wishing well or a pond.In the Middle Ages, nails were a valuable commodity that people did not waste.