An impossibly long string of DNA letters floats inside each of our trillions of cells. Somehow, incredibly, that one string produces a huge variety of tiny biological machines, from red blood cells that carry oxygen, to brown fat cells that generate heat, to neurons that fire electrical messages. How does one* fixed code manage all that?

The answer is, it’s not fixed. There’s epigenetics: In response to changes in the environment, stuff (chemicals and proteins) interacts with the DNA and affects how the code is interpreted. For example, a methyl group (one carbon and three hydrogen atoms) can latch on to the genome and help switch on or off the expression of the genes below. So in fat cells, say, some