Well, it's the side chain group that not only differentiates the amino acid from other amino acids, but it's also that side chain group that gives the amino acid its properties and its chemical reactivity. So in this lecture, we're going to focus on 15 of these amino acids. And the next lecture, we're going to focus on the remaining five. So we're going to categorize these amino acids based on their hydrophobic and hydrophilic properties based on their side chain groups. So let's begin by focusing on those amino acids that contain non polar, non reactive hydrophobic side chains. So we have eight of these.