answer: because the act of chewing on nails reportedly relieves stress, tension, or boredom. People who habitually bite their nails often report that they do so when they feel nervous, bored, lonely, or even hungry.
heart because they need it to live long.
it differ because of the characteristic they have for example volcanic have magma on it while mountain don't have magma at all.
it is because of the pressure that the mountain have for it reaches the core deeply.
Lymphatic system is anti-bodies that fights harmful organism like cancer cells, bacteria and infections that makes you sick. They'll protect you from harm.If the harmful organisms will enters your body they'll reduce the harm and makes your body stabilize after that.