Billy the dog and the invisible cat

Billy the dog was very young when he moved in with Alice.

Today was very special he was going outside by himself for the first time.

He trotted  into the middle of the small lawn, and had a good look around there was so much to see, hear and sniff.

Billy sat down, he could smell something familiar.

Suddenly a voice from behind said,  “well well what have we here”?

Without turning around Billy replied,  “Good morning Mr Cat, how are you today”.  Which is the correct way to greet a cat.

The voice said,  “Goodness gracious me”

Then after a short pause said, “You can understand me.  How is this possible young dog”?

Billy stood up turned around and looked into the face of a large ginger cat sitting on the shed roof.

Billy said   “I was born on a farm and when I was very young I was friends with Mrs Cat and the kittens.  That’s how I know what you are saying”

Mr Cat said,  “And how do you know where I am?  I thought I was invisible at the moment”.