It occurred to me that this example I've made is actually something I've seen a lot of people struggle with in their games, without ever really figuring out how to do. So this example might help more than one person! For a full explanation of how it works, read this other thread:
http://www.clickteam.com/epicenter/ubbthreads.php?ubb=showflat&Number=166388#Post166388

Feel free to use it in anything, don't need credits- it really only took me 5 minutes to throw together.