Here's why:
* Hair growth originates from the follicle: The thickness of your hair is determined by the size of your hair follicles. Cutting the hair shaft does not change the size of the follicle.
* Hair shaft is dead tissue: The part of your hair you see and cut is made of dead cells. Cutting it doesn't affect the living cells in the follicle that produce new hair.
* Trimming can make it appear thicker: While cutting doesn't actually thicken hair, a good haircut can make it *look* thicker by removing split ends and creating volume. This is especially true with blunt cuts or layered styles.
So, while cutting hair won't change the thickness, it can improve its appearance and make it appear fuller and healthier.