The daughter of a friend of mine (just a grade-school-aged child at the time) demonstrated for me how she could hold one eye completely fixed (staring straight

The daughter of a friend of mine (just a grade-school-aged child at the time) demonstrated for me how she could hold one eye completely fixed (staring straight ahead) and move the other eye independently, turning it outwards as far as she could had she been moving both eyes. The motion of the eye being moved was very smooth and deliberate. I found it absolutely fascinating and baffling. How does one even discover they have such an ability?