It started, like a lot of breakout hits, as a joke between friends. A handful of jump pads, a lava floor, and a finish line nobody could reach. Six weeks later it was the most-played new experience on the platform. The creator — still in school — says the secret wasn't the obstacles. It was the leaderboard. ‘‘People don't come back for the jumps,'' they told us. ‘‘They come back to beat their friend by half a second.'' It's a familiar story on a platform where the next viral game can come from anyone with an idea and an afternoon. But it's also a reminder of how quickly attention moves here — and how hard it is to hold.