Alice didn't think that proved it at all; however, she went on: 'And how do you know that you're mad?'
'To begin with,' said the Cat, 'a dog's not mad. You grant that?'
'I suppose so,' said Alice.
'Well then,' the Cat went on, 'you see, a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad.'
'I call it purring, not growling,' said Alice.

Author: Lewis Carroll