Issy and Jacob are walking down the street when it starts to rain, and in no time at all, it’s raining quite hard. Luckily, Issy is carrying an umbrella.
“Nu,” says Jacob. “So when are you going to open the umbrella?”
“It won’t do us any good,” says Issy. “It’s full of holes.”
“So why then did you bring it?” replies Jacob.
“Because,” Issy says with a shrug, “I didn’t think it would rain.”
From Oy! The Ultimate Book of Jewish Jokes, by David Minkoff