Why do cats lick their lips?

The cat licking his lips picture is courtesy of " Gragsie " from flickr.

When cats lick their lips it may mean that they are hungry and they tell you: "Cat want to eat…!". 

Cats lick their lips to clean themselves after eating.

Licking lips may be a sign that the cat is agitated. That the cat is in a mild state of conflict.

It may indicate there are some problems (if  they do it too much.)

Such as mild anxiety for something, an allergy or some obstruction in the digestive process.

Cats licks their lips as a displacement activity – When they are unsure, they do something to relieve the discomfort. For example, we can scratch our heads as a displacement activity, when we are embarrassed.