Plastic, glass, metal, and paper each have distinct characteristics. Plastic is a synthetic material that is lightweight, durable, and can be molded into various shapes. Glass is a hard, brittle material made by cooling molten silica or sand, and is transparent but can break if stressed. Metal is a solid material that is typically hard, opaque, shiny, and can be machined, cast, or pressed. Paper is a thin material produced by pressing together moist fibers, and is flexible, absorbent, and used for writing or packaging.