When a cat is very relaxed, the blood in a vein called the vena cava begins to form ripples where the vein narrows to pass through the liver and the diaphragm. The ripples in the vein make vibrations through the whole cat. You don't just hear the purring; you feel the purring.

Purring is a deep sound, in the range of 25 - 50 hertz. This means the sound waves vibrate 25 - 50 times every second.

Pelu was a very special friend of the Math Cats. She lived for 8 years in Spain with Boni, who has drawn some of the wonderful cats on Math Cats, including the Old Egyptian Math Cat.

Pelu passed away on February 11, 2005. But she lives on, here at Math Cats. Pelu will always purr in our hearts.