How does the "penny-taped-to- a- peice of string- trick" work?

Question

I'm talking about the trick where you take a piece of string, tape it to a penny, and let the string hang over someone's hand or other limb, and it moves in a circle if you're a girl, or a line if you're a boy...

Is there and actual reason why this works, or is this just coincidence?

Answer 1

Coincidence.  There is no correlation between this.  In my family, counterclockwise was a boy, the other way was a girl.

The body can not remain perfectly still (well, few of us can).  If you hang a penny from a string, it will eventually start to move like a pendulum because the support structure (your arm) is moving.  Tape a penny to something fixed and immobile and then hang it, it will not move.

(Exception - Coriolis force will move a mass if the "string" is long enough and the mass heavy enough - because the earth itself is moving)

Answer 2

Your hand makes subliminal motions, especially in response to suggestions, even if the subject tries to hold it perfectly still. This is a good old parlor trick for parties.