Why do cats purr?

Purrrrr ;)
Most people know/figure that when their cat is purring, it means they are content or happy. But did you ever wonder exactly why they purr?

A cat's purr is caused by their vocal chords drawing together and apart over and over again. This causes a vibration in their voice box, thus the "purr" is heard.

Reasons cats purr:

1. Communication
-Communication is the base of the answer to why cats purr. Purring is thought to be a way of communicating. Some think that when a kitten purrs, it's to reassure the mom that everything is okay.

2. To show they are relaxed
-This is one speculation because most cats purr while being petted, right? They seem very relaxed when you pet their backs.

3. To show they are injured/sick/dying
-Don't freak out and think your cat is dying every time you hear them purr, now! You'll probably be able to tell which purr is more intense than others. This kind of purr will normally be loud and obnoxious.

4. To calm themselves down
-Some people actually believe that cats purr sometimes to keep themselves relaxed under pressure. Just like all humans, felines get stressed too and it's a common belief that purring keeps cats from reaching their breaking point.

sources: http://www.petplace.com/cats/why-do-cats-purr/page1.aspx;
http://en.wikipedia.org/wiki/Purr; http://animalslook.com/why-do-cats-purr/; http://www.cat-behavior-explained.com/why-do-cats-purr.html