Rooted in an old wives' tale, the baby gender pencil test has no scientific basis, but is said to predict an unborn child's sex based on the movement of a pencil over a woman's wrist and elbow, explains Pencils.com. The movement is explained by Newton's Law of Physics wherein an object vibrates due to a displacement of its atoms, states The Physics Classroom.
With a sharpened pencil hanging from a threaded needle, the needle is poked into the pencil's eraser, notes Pencils.com. To perform the test, place one hand facing upward up on a flat surface, lift the pencil by the thread, and place the tip of the pencil on the pregnant woman's wrist. Move the pencil directly up, and try to hold it completely still. If the pencil moves vertically from elbow to wrist, the theory is that the unborn child is a boy, whereas horizontal movement across the wrist indicates a girl.
Regardless of how still a person's hand is, it is not feasible for it to obtain the equilibrium position, states The Physics Classroom. While the movement may not be visible, the energy of the pencil results in movement via air resistance and the vibrations of a person's hand and wrist. In short, it is this vibration that results in the pencil's movement, therefore leading people to speculate on a baby's sex.