The hydrophobic side chains tend to force their parts of the protein chain to fold within the nonaqueous interior of the protein molecule. These amino acids range in size from the small and barely hydrophobic alanine, which has only a methyl group for a side chain, to phenylalanine, which has a bulky benzene ring. These variously shaped hydrocarbon side chains can be thought of as the three-dimensional jigsaw puzzle pieces from which the core of the protein molecule is built. When they are fitted together in the completely folded protein, little or no empty space is left between them.