Geology
Sedimentary rock is rock that has been made from sediment.  Sediment is solid pieces of stuff that are moved by wind, water, or glaciers, and dropped somewhere.  Sediment can be made from clay, sand, gravel and the bodies and shells of animals.  The sediment gets dropped in a layer, usually in water at the bottom of a river or sea.  As the sediment piles up, the lowers layers get squashed together.  Slowly they set hard into rock.