Plastics, also called polymers, are produced by linking together small building blocks, or unit cells. Those building blocks, which chemists call monomers, are made up of groups of atoms that are derived from natural products or by the synthesis of primary chemicals from oil, natural gas, or coal. For some plastics, such as polyethylene, the repeat unit can be just one carbon atom and two hydrogen atoms. For other plastics, such as nylons, the repeat unit can involve 38 or more atoms. Once assembled, the chains of monomers become strong, light, and durable, which makes them so useful—and so problematic when they’re disposed of carelessly.