>My MacMini entertains me very frequently with spinning pizza wheels, why?
It's a common Mac phenomenan, and every Mac user is likely to experience it at some point when using OS X. It's basically a cursor for when the program you are using no longer responds.
So why does the application not respond? Usually, it's busy. Often it takes time to swap memory or process something, so you have to be patient. Or if the program is badly written, it may have crashed (even well-written programs crash).
>How do I fix.
Usually the short-term solution for me is simply to reboot my machine. It ensures a flush of the memory, allowing the Mac's processor to start anew. The longer-term solution is to buy more RAM, but it all depends on what applications and how many you're running.
If this never occured before, and you've always used the same number of programs, consider a reinstall as your filesystem may have gotten a bit scrambled.