When we humans are happy and feel good, our eyes light up and our mouths smile. When we are really happy, we open our mouths and laugh out loud.

However, the cat does not move the corners of its mouth, regardless of its state of mind. You can therefore not tell from the cat's mouth whether it is happy. So how does a cat actually smile?

When people laugh and are happy, more air enters the throat through the open mouth, which causes the vocal cords to vibrate. This vibration combined with the "acoustics" of the oral cavity produces the sound of laughter.

When you stroke a cat and it feels good all round, it starts to make what we call a "purring" sound. It sounds like a motor chugging along quietly, and if you touch the cat's throat with your finger, you can feel a vibration. Cats mainly use body language to express their state of mind. They rub themselves devotedly against your stroking hand when they are happy, they wag their tails when they are concentrated and they ruffle their fur when they are angry. In addition to body language, they also use sounds to communicate. They have a range of meow sounds depending on what they want to say, they hiss when they want to chase away an enemy or they purr.

The purring is caused by the cat letting more air in through the nose via the throat when purring, which triggers a vibration of the vocal cords. The vibration produces the purring sound. So basically it's exactly the same as human laughter, except that the cat is able to vibrate its vocal cords many times faster. In other words, if we humans were able to vibrate our vocal chords as quickly as a cat, we would actually purr. Purring is like laughing at a higher frequency.

We know that a cat purrs when it feels well. However, it also happens occasionally that a cat purrs when it is ill. Purring releases happiness hormones in the cat's brain. It is therefore assumed that the cat purrs even though it is ill in order to release these happiness hormones, make it feel better and speed up the recovery process. People often try to distract themselves with funny films when they are ill for exactly the same reason. The motto for both humans and cats is: laughter is the best medicine.