Humans use 20 different amino acids as building blocks to make the thousands of proteins found within the body. Each amino acid has the same basic structure with a central carbon atom that has four groups attached. Three of the groups are the same in every amino acid: one is a single hydrogen, one is an amino group that acts like a hook, and one is a carboxyl group that acts like an eye. The fourth group is a side chain, which is unique for every amino acid.

Each side chain has a unique size and shape: some side chains are round and bulky, some are short and skinny, and some are longer than others.

Unlike the carboxyl and amino groups, which always like a watery environment, side chains vary in their chemical properties.

--Some side chains have a charge, and that makes them prefer to be in water.