Plastics are based on molded organic polymers that are usually of high molecular weight. There are numerous sources both natural and synthetic for these polymers. Casein is a common family of proteins in milk. It makes up to 80% of the proteins in cow's milk. Casein was a source of organic polymers in the early days of plastics in the early part of the 20th century. In particular, casein and formaldehyde were used to produce a plastic called galaith. This plastic is relatively brittle and cannot be molded once the reaction was set, thus limiting its uses. It was cheap to produce, however, and could be used to make thin plastic sheets for punching out buttons for use in making clothing. But outside of buttons, there was not much use for casein based plastic. Petrochemicals are currently the main source of polymers for modern day plastics.