Every hair on your head or on your body is made up of a root, the soft, bulb-shaped section under your skin, and the shaft, the longer strand that sticks out of your skin.
If you were to cut a shaft of your hair from near the root and look at a cross-section of it under a microscope, it would have either of two shapes: round or flat. If it appears round, that means your hair will grow out straight; if that cross-section appears flat, that means your hair will grow at different rates in different places, making it curl or wave. The flatter the shaft, the curlier it will be. The shape of your hair shafts is determined by the shape of your parents’ hair shafts, for this is one of the characteristics you inherited from them.
Some people with curly hair are unhappy with it and have their hair straightened, while others with straight hair take “permanent waves” to make their hair curly. But both of these changes are only temporary, lasting only a short time, for the shape of the shaft cannot be changed, and as that person’s hair grows out, it will look the same as before the change.