A bow is a weapon designed to shoot an arrow. Historically, the bow predates written history and is characteristic of most cultures. It is believed that the bow as a weapon appeared in the Old Stone Age, 8 thousand BC. Rock paintings and finds of arrowheads from this era outline the historical boundary of the appearance of the bow. The oldest find of a bow is in a bog in Denmark and is dated around 6 thousand BC. It is a simple wooden bow, very similar to later medieval longbows. Shooting takes place at the expense of human muscle power, which the bow accumulates in potential energy at the expense of the bending parts, and then, when the string is released, converts it into kinetic energy and gives it to the arrow. One of the main materials for making a bow is wood. In different eras, in different cultural traditions and geographical regions, different types of wood have been used. The most commonly used types of wood are ash, yew, cedar, birch, hazel, maclura and a number of tropical species. There are bows made entirely of horn, using tendons to strengthen and/or connect the individual parts. In Asia, for example in India, bows made of metal were also used during the Middle Ages.