SERIOUSLY THIS IS any one know 'Just what is a cats Purr? is it their heart ? Why can they turn it on and off?

One hypothesis, backed up by electromyographic studies, is that cats produce the purring noise by fast twitching of the muscles in their larynx, which rapidly dilate and constrict the glottis, thus causing vibrations in the air both during inhalation and exhalation. Combined with the steady inhalation and exhalation of air as the cat breathes, a purring noise is produced with strong harmonics.

No one knows for sure, though they've been studying this for years. The first answer is considered one of the best theories; as for why cats purr, it usually expresses contentment, but it can also signal pain (the cat purrs in an attempt to soothe itself), or fear (the cat purrs to signal that it's not a threat).

I am not sure either however I had one cat that never purred. When I took it to the vet for a checkup I mentioned it to the vet that the cat was very happy and friendly but never purred. The vet couldn't explain it either and said maybe it was just something that the cat couldn't do.

same as the first answer but i think its adorable and i love it when they do that!