According to animalplanet.com when cats are relaxed and happy, the muscles of the cat's larynx (also called the voice box) act as a valve for the air they breathe, causing them to vibrate as they breathe in and out.  That's what makes the purring sound.  It's a bit like the muscle reactions we have when we are happy:  when a human is happy, we smile or laugh.  Cat's don't have the same control of their face muscles as we do, but they have control of their voice box that we do not.  Other creatures purr, too, such as bears, racoons, and badgers (for more animals that purr, see this article at Wikipedia)! 

For many years it was thought that lactic acid that builds up in your muscles as you exercise was what caused them to be sore 24 to 48 hours after a workout.  However, scientists have found that this is not the case at all!  Actually, your muscles use lactic acid for fuel during your workout, and so the buildup of lactic acid only lasts a few hours, so naturally this cannot be the reason why your muscles hurt 24 hours later.  

The real reason you are sore is that as you exercised, you actually made very small tears in your muscles. 
(Don't worry, these tears are extremely small, and are not dangerous.  They are just part of how your muscles get stronger!)  As your muscles are healing these tears, they swell up and experience inflammation, which is where the pain comes from.  If you do this kind of activity often, your muscles will adapt to the effort, and the tears will not occur, so you won’t feel sore.  

On a final note, the pain from tiny muscle tears that mean you are getting stronger should always occur 24 hours or so after you exersise.  If you find this pain occuring during your exersise, you should talk to your
doctor.