A helicopter dad nearly spins out of control as his sons struggle to master the knuckleball—and perhaps save baseball’s most storied pitch from extinction.The knuckleball is the only skill that can elevate an athlete without a superjock’s speed, strength, or explosive power to the Hall of Fame. Thrown without spin, it darts around unpredictably for reasons physicists and engineers labor to explain. It’s a secret weapon every pitcher should want to master, so why is...