But what exactly is evolution and how does it work? Evolution "is the net change in organisms or a population over the span of many generations." We can break this definition into two parts - the net change in organisms or a population, and one that occurs over the span of many generations. The first part reveals what and where these "changes" happen - the mechanisms of evolution like natural selection. The second part tells us two things – the time it takes for organisms to change, and, less obviously, that the process involves reproduction, DNA, heritability, and change. The core of evolution is the change in organisms, and the first place to investigate this change is in the DNA. DNA is the blueprint of all life. Its sequence is composed of four letters that bond in specific ways: A to T and C to G. Small sections of the DNA sequence, called genes, code for certain traits like eye color or blood group. Genes can come in a variety of "flavors", meaning that the gene for eye color can code for blue eyes, or brown eyes, or even green eyes! Each of us possess a unique combination of these genes, giving some of us brown eyes with blond hair, or blue eyes with brown hair.