Zach gets his first microscope for Christmas, when he's ten, and he can't be torn away from it. He usually plays in the snow with his siblings, or flails as they team up against him in snowball fights, or at least sits and somberly informs everyone of the inaccuracies prevalent in the small nativity scene in the family room (he's done research, he knows that's not the way things looked in the Middle East circa 3 B.C.). Instead, he spends the next weeks ensconced in the little set of slides that came with the microscope, examples of fungi and bacteria already on them, and later he starts to explore the area around him with little plastic slides and a lot of patience. Gotta catch them all.
When Meghan scrapes her knee and starts picking at her scabs like she always does (much to the bereavement of their mother), he wheedles one off of her in order to study it under his microscope. When he falls and scrapes his knee, he takes it as an opportunity to study human blood. The family dog is assaulted so that he can look at the differences between dog hair and human, the chickens for similar studies of feathers.
When spring comes he goes to the pond because he heard that it was full of tiny little creatures (unicellular flagellates, amoeboids, and more) . His parents are still telling themselves that this is a "phase," and he'll be back to running around pretending to be a horse with his sisters soon. Even if they do insist his name is Rainella Princess Sunbeam Moonshine.
When he's twelve he saves up his money and buys a new microscope, on the recommendation of the biology teacher. Several months later the teacher calls his parents and asks if, exactly, they know exactly how advanced their son is—and they say, of course, he's a sophomore in high-school at the age of twelve. It's the opinion of all the teachers—he's in senior calculus at the time—that he should be in college by now, but his parents balk at that. He gets sent to an elite private school, known for its college-level classes, though, the sort no one expected the son of a normal Midwestern family to go to. He has a full scholarship. He's the talk of the microbiology teacher, full of insights and quick leaps of judgment. He's the talk of the math and economics teachers, when he comes to talk to them about game theory in their offices. He doesn't talk to anyone else.
And then, he looks up from his microscope—metaphorically—from his long complex equations he was doodling on the back of his notebook—literally—on the first day of anatomy and physiology, and he never really looks back down.