I was wondering what "drop the penny", "help get the penny to drop", or things similar mean? All I can understand is that it must be a metaphor. For example:
simply trying to repeat things in perhaps a slightly different way may help get the penny to drop (think about the student coming to office hours who has read the book but doesn't get it; just going over it again in different words can help).
How to use such an expression in life generally. Can you give other examples?