Why do cells with the same DNA have different functions?

Almost all of the cells in your body share the same DNA as was found in that first cell * ... Eyes and lungs are so different because different sets of proteins are made in the various eye and lung cells. In other words, in the eye, genes are turned on for seeing and the "breathing" genes are off.

In humans, nerve cells and liver cells have the same set of DNA, yet their functions are different. During this period each cell turns on the specific set of genes that will enable it to become the specialized cell type it's going to be.