Amino acids are the building blocks of proteins. And inside our body, we have 20 different types of amino acids. Now, what exactly distinguishes one amino acid from another one? 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.