The Penny Drops.
I saw the explanation here but heard a totally different explanation. An old courting tradition for young ladies was to sit with a penny held between their knees while in the company of young gentlemen. If the young lady's mind wandered from the chastity exercise the sound of the penny dropping was heard by chaperones. Probably connected to the expression "keep your hand on your ha'penny". May well be nonsense but thought I'd post it.