Hair form depends on hair follicle form and its locality in the skin, the keratin composition in hair core, the individual race and characteristics. Yet we can say that the hair form depends on its root form: if the follicle has a straight shape, the hair has straight form (1), if the follicular form of a crescent, hair grow wavy (2), if the follicular form of a spiral, the hair will be curly (3), so as hair forms in hair follicle, it takes a hair follicle form. Sometimes different parts of the head may have various forms of hair, in addition there are cases, when a sudden change in health or weight may provoke changes in hair form. However scientists still do not fully understand why the different types of hair grow straight or curly or wavy. This is probably determined by several factors, which may vary in their importance during life. This is why some curly-headed children have straight hair later in life, and vice versa. These factors include: the way the large bundles of keratin are arranged within each hair shaft the position of the hair bulb in the hair follicle - in African hair the bulb may lie to the side of the follicle, and so the hair shaft grows out of the follicle at an acute angle irregular growth in the hair bulb - if it varies slightly to one side or the other the hair may grow wavy the shape of the hair follicle, whether it is straight or curved the number of twists per unit length. All hair, even apparently perfectly straight hair of Asian people, twists as it grows. The number of twists in a given length of hair determines how curly it is: the more twists there are, the curlier it will be. Some African hair has 12 times as many twists per centimetre as Caucasian hair. In Asian people the keratin bundles in the hair are all straight. The hair shaft tends to be thick, and almost completely round. The keratin bundles in the hair of Caucasoid people are a mixture: most are straight, some are wavy. The proportions of the two types vary a lot. The hair shaft is usually oval in shape. The tightly curled hair of African people twists much more frequently than in the other groups. The hair shafts are markedly oval in shape, with definite edges. The cuticle is sharply kinked at the edges, and is especially easily damaged at these points. This curious shape is the reason for the vulnerability of African hair to all forms of physical and chemical trauma, and its consequent need for extreme care in handling and very thorough conditioning. Straight hair sometimes becomes quite wavy when damp, and wavy hair may become straight when thoroughly doused with water - in a swimming pool, for instance. This is because in these conditions great numbers of hydrogen bonds have been broken. The effect is very temporary, and the hair recovers on drying. Over many thousands of years these three groups of people have intermingled. Their descendants now show every imaginable blend of hair type and color.