How a purr is made
A purr is produced deep in the cat's throat: rapid signals from the brain make the muscles of the voice box open and close the space between the vocal cords many times a second, around 25 to 150 times per second. Because this happens on both the in-breath and the out-breath, the purr can roll on almost continuously, which is unusual among animal sounds. It is a remarkably efficient little motor that a cat can keep running for a long time with very little effort.
Not always a sign of happiness
We tend to read purring as pure contentment, but the picture is more complex. Cats purr when they are relaxed and being stroked, yes — but they also purr when stressed, frightened, in pain, at the vet, while giving birth, and sometimes even when dying. Researchers believe purring can be a way for a cat to calm and comfort itself in difficult moments, not only an expression of pleasure. So a purr is best read together with the rest of your cat's body language and the situation.
The purr that gets your attention
Some cats have a special "solicitation purr" they use to ask for things, often food. A 2009 study (McComb and colleagues) found that this purr hides a higher-pitched cry within it, acoustically similar to a human baby's cry, which taps into our natural urge to respond and nurture. People in the study found this purr more urgent and harder to ignore than an ordinary purr. If your cat seems to have a particularly persuasive breakfast purr, this is likely why.
The healing question
You may have read that the low frequencies of a purr help heal bones and tissue. It is a genuinely intriguing idea, and the frequency range does overlap with vibrations used in some medical therapies — but it remains an unproven hypothesis rather than an established fact. What we can say for certain is that purring is a rich channel of communication and self-comfort. Enjoy it for what it clearly is, and remember that a purring cat is not automatically a healthy one — always judge wellbeing by the whole animal.