Even a seasoned baseball pitcher has difficulty throwing a ball to the same location repeatedly. I will talk about how such movement variability comes from muscles that activate at imprecise times, and introduce a new method that uses a smartphone to robustly quantify the movement variability, which is tightly related to the arm muscles’ timing precision. This handy method reveals how movement variability, a measure of dexterity, changes with growth and age, and enables us to quantify the degree of handedness and footedness.