Other important classes of substances containing very large molecules are the plastics and artificial fibers, which are such a conspicuous, though not always a positive, feature of modern life. Most of these materials are made in the same basic way. The starting materials or monomers are relatively simple molecules—usually carbon compounds derived from petroleum—which can be persuaded to link up with each other in order to form a long chain of repeating units called a polymer . If we think of the monomer as a bead, then the polymer corresponds to a string of beads. Polymers are usually classified into two types: addition polymers and condensation polymers , according to the kind of reaction by which they are made.