Apple

Nothing much you can do about it. Over time the keys collect oils from your fingers. For some people it's not as much of an issue but if you eat while using your computer, rarely wash your hands or have sweaty hands, this will happen quicker. Turn the machine off, get a microfiber cloth and some eucalyptus oil and dapen the cloth with it and wipe the keys. Make sure the cloth isn't wet so that any moisture gets into the keyboard area.