The bulb! I hope this helps :)
Explanation:
a disorder of structure or function in a human, animal, or plant, especially one that produces specific symptoms or that affects a specific location and is not simply a direct result of physical injury.
Symptoms of depression include having a loss of energy, change in appetite or weight particularly noticeable weight gain or loss, trouble focussing, feelings of hopelessness, and loss of daily interest in activities.
Nail biting happens Because people have anxiety. The nail biting relieves stress, tension, and or boredom.
Hope this helps
~QueenSupreme aka Black-Rose♥