Common rubbers, such as polybutadiene and polyisoprene (also called natural rubber), are produced by a process called polymerization. The process starts off with very long molecules (polymers) that are built up sequentially by adding short molecular backbone units through chemical reactions. A rubber polymer follows a random, zigzag path in three dimensions, intermingling with many other rubber molecules. An elastomer is created by the addition of a few percent of a cross linking molecule such as sulfur.