What is grandma's on the roof story?

This guy's parents are away on holiday.

His father phones up to ask how things are going. The son replies "the cat is dead".

The father gets a real fright and is really upset. So he says " Son, you could have put that a bit more subtly. For example, you could have said that the cat climbed onto the roof. He slipped, and fell. And oh, no, he hit the ground and died".

Anyway, how's grandma?

Son replies... "grandma's on the roof...."