Why are horseshoes believed to bring luck?

In 18 century a Frenchman Henri Mission, who was visiting Britain, asked villagers why they had horseshoes nailed above their doors. He was told that it was to keep witches away. Horseshoes are made of iron, so the strengh of the iron is believed to protect from evil. Still today they are thought to bring good luck, but it's very important to position it upwards like a cup, so that the luck wouldn't fall out.