How dome some cells become brain cells and others become skin cells, when the dna in all the cells is exactly the same. in other words, if the instruction are exactly the same, how does one cell become a brain cell and another a skin cell?

All multicellular organisms develop from a single fertilized egg cell known as a zygote.  As this cell divides by mitosis, each new cell that is formed contains the exact same genetic material.  As the organism develop certain genes in certain cells are activated and others are inactivated so that cells develop into specialized cells such as brain cells and skin cells.