A woman telephoned a veterinarian and asked him to come examine her cat. "I don't know what's wrong with her," the woman told him. "She looks as if she's going to have kittens, but that's impossible. She's never been out of the house except for when I had her on a leash." The vet examined the cat and said there was no question about her pregnancy.
"But she can't be," protested the woman. "It's impossible."
At that point a large tom cat emerged from under the sofa.
"How about him?" asked the vet.
"Don't be silly," answered the woman. "That's her brother."