Chinese martial arts, also referred to by the Mandarin Chinese term wushu and popularly as kung fu, are a number of fighting styles that have developed over the centuries in China. These fighting styles are often classified according to common traits, identified as "families", "sects" or "schools" of martial arts. Examples of such traits include physical exercises involving animal mimicry, or training methods inspired by Chinese philosophies, religions and legends. Styles which focus on qi manipulation are labeled as internal, while others concentrate on improving muscle and cardiovascular fitness and are labeled external. Geographical association, as in northern and southern, is another popular method of categorization. Bone carving is the act of creating art forms by carving into animal bones and often includes the carving of antlers and horns. It can result in the ornamentation of a bone, or the creation of a figure. It has been practiced by a variety of world cultures, sometimes as a cheaper, and recently a legal, substitute for ivory carving. It was important in prehistoric art, with notable figures like the Swimming Reindeer (antler), and many of the Venus figurines. The Anglo-Saxon Franks casket is a bone casket imitating earlier ivory ones.