What causes a hairy tongue?

Can be caused. By a host of reasons - typically lack of proper hygiene - stimulation of the dorsal surface (top) of the tongue. Can also be due to medications, smoking, heavy coffee and tea consumption, etc. It is a benign, harmless, painless condition. It can take on different appearances (colors).

Related Questions

What can cause a white hairy tongue and red bumps?

Multiple causes. Nutritional deficiencies, reduced health status, immune compromise, overgrowth of candida albicans, exposure to wide spectrum antibiotics are potential causes. The red spots are probably taste buds, the white parts foliate papilla, parton the tongue that is normally pink. Read more...
Some are normal. Each person's tongue is different, some can be red, whitish, cracked etc. If there is a bump on both sides, usually it is normal, anything that is sore or irritated should be looked at by a dentist. Read more...