What six things that decay can you put in compost piles?

Cardboard, egg shells, fruit peels, grass clippings, leaves and paper are six things that decay and that may be put in compost piles. Fruit peels and grass clippings decompose within six months while cardboard and leaves -- excepting beech and oak -- require 12 to 24 months. Egg shells take at least three years to decompose.