“You ready?” the hairdresser asked me with a smile, holding foil and a tub of light cream in her hands.

This was supposed to be fun, I told myself. Staring in the mirror, surrounded by scissors, blow driers and chemicals, I paused. I had always been brunette. Why did I so desire a change? I’d just wanted to try something new, different. Why was I suddenly hesitant?

The pungent stench of the bleach raked my nose. I knew that once the hairdresser brushed the mixture, containing the same chemical used in fertilizers and household cleaners, onto my hair, the molecules in the bleach would begin permeating every brown strand. The hair’s walls must first be penetrated. When the chemicals gained access to the depths of my hair, they would rob the color from the hair’s proteins, leaving only colorless molecules in their wake. Foreign molecules of a new color would then take up residence within the hair shaft while the outer wall of the hair remained open. Those strands of hair would transform at the molecular level, never to return to their former state.

The dye solution is most effective when first mixed. As time goes on, the chemicals steadily decompose as they react with each other and the oxygen in the air, their ability to add color to hair decreasing. The hairdresser had already mixed the solution. Her foot tapped the floor.

The dark brown color of my hair is a present from my parents. My mom and dad were both brunette, blue-eyed, and on the short side. I have all of these characteristics, along with my mom’s legs and my dad’s propensity for being late to things. The fact that we are each a particular blend of genes, with certain fixed features that will never change, is, to me, at once beautifully reassuring and mind-blowingly frustrating. The many genes within us form the blueprint for how we will look, how sharp our eyesight will be, what diseases we may be prone to, and even some aspects of our behavior. Our unique concoctions of characteristics make us special, irreplaceable. Yet in a way, though we can alter small things about ourselves, we are stuck in the mold we are given. I will never know what it’s like to be tall, male, dark-skinned, or a squirrel. I can pretend, I can ask, I can watch. But I will never truly be any of those things. The unattainable can be maddening.

Scientists, psychologists, and self-help book writers quibble over how rigidly our genes define us. Much evidence points to fixedness. Identical twins adopted at birth and raised in separate homes have remarkable similarities, enjoying the same hobbies and school subjects, despite never speaking to one another. Health and personality traits of adopted children raised together in the same homevary just as much as they do in separately housed, unrelated children. Medical news articles repeatedly blare your likelihood to get high blood pressure (1 in 4 if you are Latino) or Type 2 diabetes (1 in 7 if your parents had it) based on your family history. A forty-year vegan dies of a heart attack. And I have terrible eyesight despite years of ensuring proper reading light. In these cases, nature seems to win the war.

There are some changes we can make, though. A different hair color is an achievable and reversible change. Cut and color are no longer just hair terms; people today can change their nose, eye color, breast size, and body shape. On my most recent birthday, the reminder of years passing turned my thoughts to things I’ve done and those I have yet to experience. I’ve tried camping, horseback riding, skiing, Disney World, Paris, poutine…I have a list. For my previous birthday, I’d gone skydiving to learn how it feels to fly, to be a dive-bombing bird or an asteroid hurtling toward Earth. Dying my hair was also on the list, and the thought of leaving this world before going blonde was suddenly torturous. I had to know. The hairdresser reassured me, for the fifth time, that I was right to feel this way.

I thought again of my genes. Within me, there are a number of genes that determine my hair color; I know the most assertive gene calls for brown pigments. But there could be a blonde-haired or a red-haired gene sitting there, passively, passed down from my great grandmother or grandfather, and I would never know. I like to think that I’m full of secrets.

The number of potential secrets seems high, at least. The grand endeavor from 1990 to 2003, the Human Genome Project, counted, analyzed and compared our genes and told us that there are roughly twenty-five to thirty thousand of them in each of us—although that’s not much more to work with than a chicken has, and less than a grape plant. How many secrets must a grape have? I often wonder about these secrets, the unexpressed genes. Some will never reveal themselves; others may one day activate, brought to life by some meddlesome external force or mysterious inner timer. In any case, I await surprises.

Over time, even identical twins develop differences, separate diseases, alternate personality traits. The world pulls their genes apart. The cells that make up each facet of our bodies are continuously changing, dying, renewing. Even my hair will one day surprise me. My hair genes will permanently order production of brown pigment, but eventually, the pigment-producing cells will tire out, no longer able to carry out the gene’s orders—and my hair will turn gray.

We go on as we are, growing and maturing, yet we are not made up of the same stuff today that we were when we were born. Skin cells live only two to three weeks; colon cells four days. Hair is made from stem cells, which have the enviable ability to physically transform into another kind of cell. They can remain a stem cell, or they can become a brain cell, a skin cell, a red blood cell. They can divide endlessly to replace other cells and repair parts of the body when they need it. They are the utility players of our bodies. The myth that we replace ourselves every seven years is not entirely true, but it relays the right idea; underneath our seemingly fixed forms, there is a world of constant flux.

Our brain cells—neurons—on the other hand, can last as long as we do. These stick with us, keeping our lists of things we think our body should experience. Yet they too embrace change. Neuroscientists long believed that brains stopped developing entirely after a certain time period, but we now know that our brains have an exceptional ability to change at any age. The neurons themselves may not transform, but they can learn to form new connections with other neurons by “re-wiring,” and even alter the structure and organization of the brain.

Neurons fired, wired inside my brain, and a thought formed. Perhaps I could turn blonde for a while, but deep down, in my genes, I would never really be blonde. Just as I could never become a basketball player, or Emma Watson. I can transform in other ways. Indeed, I will be constantly changing, whether I like it or not.

To my hairdresser’s annoyance, I chose to keep my hair molecules brown.