I don't get that either

please explain

the English have lots of old out dated " sayings" ( as you probably found out already)this is another one of those sayings.
if someone does not know what someone is talking about and has a blank look on their face.you can see them thinking about it.the expression on their face changers and you just know they suddenly know what you mean. "the penny has just dropped."
where this comes from ,like most stupid sayings i have not a clue.

what does the penny dropping have to do with taxis, drugs, and women

because i did not understand it at first and after a while i got it.

so can you explain to me what the title means?

** info **

http://www.takeourword.com/TOW186/page2.html

thanks for that bamber.i knew you would come up with the explanation.

but no one explains the how the title connects with the picture!