Do you know why some people have curly hair and some straight? Well, curly or straight, it all depends upon the number of chemical bonds that is present between hair proteins that are found in the hair shaft. Greater the number of links, curlier the hair, and fewer the number of links, straighter the hair! The amount of humidity in the air also alters the degree of curliness or straightness of hair. High humidity forces water into the hair fibre and forces the hair shaft to return to its original structure.