There was a man whose wife is about to deliver a child. He wanted a Boy child & he was quite confident that she will deliver only Boy child. So he went along with his wife to his in-laws place & stayed there.

He stayed there for more that 15 days, but she didn't deliver the child at all. So he went back to his place. Before leaving he told his father in law that when she delivers the child just call up & tell me that "the clock has arrived" & that is the code word to understand that boy is born.

Next day she delivered a girl child. The father in law didn't know how to inform his son in law. He can't say that the Clock hasn't arrived also.

So he calles up & said, "The Clock has arrived but the pendulum is missing"