Why do i chew on stuff?

i chew on rubba band, wood, tootpicks, soda canz, paper, eraser, hair, wires, food, gum, new shoes, i eat crayola clay , i chew pensil , crayon, bed covering,foil, plastic bags, garbage bages, glue bottle tips, buttons , aglets (plastik tipz on shoolaces) nails ,nail polish, paint chips,i eat powder, thick text books,sillly putty, ds styluses ...