A woman takes her cat to the vet, and the doctor says, "Your cat is pregnant."

The woman says, "Impossible, it has never gone outside."
The doctor says, "Then maybe your window wasn't closed."
The woman says, "I live on the 18th floor."
The doctor says, "Then maybe a male cat climbed up."
The woman says, "How can it climb on the 18th floor?"
The doctor says, "Then maybe you have another cat at home."
The woman says, "Just this one."