Four years ago, Aly Raisman was on top of the world. She captained the U.S. women’s gymnastics team at the 2012 London Olympics, snagging three medals — in addition to a team gold, she earned the distinction of being the first American woman to top the podium for a floor routine. She had been told that landing her opening tumbling line was impossible. She did it, anyway.

This week at the Olympics in Rio de Janeiro, Raisman led the American team to gold again. On Thursday, she performed a routine deemed harder than her 2012 performance — but earned a silver. Raisman has said silver was the goal: Her teammate Simone Biles is nearly untouchable. “All, the girls are like, `Simone’s just in her own league,’” Raisman told USA Today. “Whoever gets second place, that’s the winner. Simone gets her own super first place.”

How does a routine go from “impossible” to distant-second in such a short period of time?

Let’s start with the mechanics. Raisman’s routine — a seven-second tumble, a roundoff, one-and-a-half stepout, roundoff, back-handspring, Arabian double front, punch layout — is difficult not by measure of any component part, but by the combination of elements crammed together. The most impressive single skill, the Arabian double front, involves launching yourself backwards and upwards while turning 180 degrees in the air, then completing two forward somersaults in the air before landing. It’s absurdly hard, but many elite gymnasts can perform it consistently in competition.

It’s the addition of all that other stuff, pushed together in a limited floor space, that makes for such an extraordinary challenge. “It’s the hardest part of my routine just because I have to worry about fitting the whole thing into the floor; sometimes I go out of bounds,” she tells the Wall Street Journal.

Biles doesn’t worry about going out of bounds when she performs “The Biles,” her signature move where she flips herself backwards twice with her body fully extended, ending the move with a half twist before landing. She’s the only person in the world to land the move in competition. Because she’s getting in an extraordinarily difficult single move, she worries less about smashing other complex moves together in combination.

Biles takes three steps into her roundoff, then builds up momentum over two back handspring before launching in the move, and landing with room on the mat to spare. Raisman gets only one step, and she has to make sure it’s a small one if she’s going to end the set within the legal boundaries.

Speaking of boundaries, pushing the limits of the possible has been the norm in women’s gymnastics for the entire history of its existence as a sport. Don’t forget the Olympics are a big business, and where there’s money, there are incentives to push athletes farther. That means better, smarter coaching and training. It also means better equipment: You can bet that Raisman’s routine would still be “impossible” today without springy floors that allow for higher jumps and softer landings.

The rules have also changed to favor athletes who push themselves higher. Half a century ago, floor routines looked like exercises in dance, emphasizing style and grace over acrobatics. The change towards high-flying tumbling lines has come gradually, but it was pushed by an overhaul of the judging system in 2006 that sought to maximize objectivity and minimize scandals. The result is that points are awarded for quantifiable things like technical difficulty, more so than subjective elements of style.

Simone Biles is amazing, but don’t make the mistake of calling her untouchable, or her moves impossible. Somewhere, the next iteration of Simone Biles is tumbling and watching, waiting to make the Biles look like the easiest thing in the world.

Photos via Getty Images / Laurence Griffiths, NBC/YouTube

Jacqueline Ronson is a science writer based on Vancouver Island, Canada. Before that she lived way up in Whitehorse, where she reported for the Yukon News. These days she likes to talk to smart people about the future of the planet, ride her bicycle, play her banjo, and frolic.