A good answer is truthful and understandable for the child. Tell that cats can be happy, sad, afraid, and experience different emotions. They do not know how to laugh, cry and talk, so they communicate in a different way. With a purr, the cat "tells" how it feels. Pleasant sensations are usually implied: the cat likes being stroked; she is glad to return home from work or from a walk; she was treated to her favorite delicacy. This purr "translates" from feline language as "hurray, great, I'm very happy." This explanation will be enough for a toddler who only interacts with pets under adult supervision.
Explain to your older child that sometimes cats purr when they are very scared and / or in pain. Anxious purring is distinguished from a joyful animal in behavior. If it huddled in a corner, hid under the bed, does not respond to its name and purrs - the cat is not all right, it may have gotten sick. You need to call adults.
A preschooler may wonder why cats can't pronounce human words. Tell him how the person speaks and the animal makes sounds. Commands come from the brain, they are executed by the tongue and vocal cords. The human brain gives commands to reproduce words, the cat's brain - meows and purrs. Cats have an undeveloped part of the brain responsible for speech. Another reason is the structure of the larynx and tongue that is different from the human. A cat is physically unable to pronounce most of our sounds, their combinations and words.
http://www.youtube.com/watch?v=5dJfUAe9d2Y
