If you look in your Mac help, then search for screen capture, you'll see that one of the choices will show you a list of the shortcuts for the variety of screen captures possible in OS X. They are also made through the Grab utility (in your Applications/Utilities folder). If you open that Grab utility, you can look through the menus there. The shortcuts used vary slightly depending on which version of OS X that you have, so the Grab utility will show you exactly what keyboard shortcuts you need to use, and how to use them.
Serendipity is a lucky guess !