FLTK puzzle

[boson]

Hello there,
Here is the thing,
I am tring to teach myself a C++ from Stroustrups Programing principles and practice using C++. I have been doing rather rapid progress, as I did have some experiences from school, although I havent writen any code for few years. I have discovered the burned out ashes of what once was a great passion of mine, and mannaged to get the fire going again.
I got stuck at this FLTK excersize. My policy is to do as many exersizes as possible to make sure I really mastered new topic presented in each chapter. There is this one excersize. It requires building a picture of clock and geting a callbacks from FLTK each second, witch will move my clock. I am aware the fltk has got a clock widget, but I dont wanna use it. I know how to draw and move object in a window, but I am clearly missing something fundamental here. I cant figure out how to implement callback with Fl::run() and Fl::wait(). where and how should be the reqest for callback inbeded in the code. What should recieve it ?
I have been stuck on this for a month now. Somebody please help !
thank you