For Wolfe-Simon, the question was, How flexible is life? Every known living thing on Earth shares certain chemical and biological characteristics: They all ingest some form of energy, use it, and release it in a different form. They all use the same 20 amino acids to build the proteins that enable that activity, and they all use DNA and RNA molecules to store genetic information. And as far as scientists have found, they all require six elements—carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur—to survive. Within those constraints, though, are many variations. For instance, our blood contains iron—that's why it's red—to ferry oxygen around, but blue-blooded crustaceans use copper. Most living things use oxygen to burn sugar for energy, but some, including many bacteria, use nitrogen or sulfur.