We've all heard cats meow and hiss, but why do they purr? There are a number of theories out there - from the commonly held belief that they are simply happy, to British comedian Eddie Izzard's light-hearted suggestion that they are impersonating a drill.

While they do purr when tickled or caressed to show their appreciation for attention, cats also make the noise when not in our company. The purring noise is a consistent pulsing sound that occurs while they both inhale and exhale breath. The word 'purr' is a great example of onomatopoeia and refers to the constant noise which ranges in frequency between 25 and 150 hertz.

For a long time, scientists debated how cats purred. Most people believe that the cat's brain signals the laryngeal muscles to vibrate and function like a valve, opening and closing the space between the vocal cords. What triggers this reaction is debated. Some argue that the cat intends to purr, while others say it may be due to endorphins released by the brain as a reaction to pleasure, or pain.

According to Leslie A. Lyons, an assistant professor at the School of Veterinary Medicine at the University of California, cats often purr while stressed, such as when they head to the vets or are recovering from an injury. So what are the potential benefits of this 'drilling' sound? Some scientists believe the vibrations that ripple through the cats' bodies may actually promote muscle development, heal bones and injuries, ease breathing and lessen pain. It could even help to soothe discomfort in a similar way to a child sucking their thumb.

So, the purring of a cat may have more uses than just telling you they're enjoying attention. It's also important to remember that not all cats can purr. Some bigger felines, like lions, can't, and only roar. So, the next time you hear a roar, it probably isn't your pussy cat, and you might need to run.