This old wives' tale operates on the theory that an unborn child can exert some sort of control over objects held outside the womb. Perhaps it's teeny-tiny telekinesis? Anyway, the "test" can be performed with a wedding ring, pin or needle tied to a string or strand of hair. Mama-to-be then lies on her back (cue dirty jokes about how that's how she got into this mess in the first place) and someone dangles the laden thread over her belly. If it swings in a circular direction, she's having a girl. If it moves side to side, it's a boy.
Herein lies my main issue with this practice. It's really easy to manipulate the string, unconsciously or not, to achieve a particular gender result. (It's called the ideomotor effect.) As hole-filled as this particular theory is, it's still a harmless way to revel in the joy of impending motherhood – and can be a fun baby shower game. Just don't go crazy buying pink or blue baby swag based on the results!