There was an expectant father who had spent quite some time waiting for the offspring to arrive - at his in-laws' place.
As his leave balance had gone into the red, he tells his father-in-law, "When my son comes, do not call up my office and say that I have become a father of a boy because I'll have to shell out a lot for parties. Just leave me a message that the clock has arrived. This will be our code for the arrival of the baby."
The offspring does finally arrive one day, but it's a daughter.
The father-in-law now thinks to himself, "If I tell him that the clock has not arrived, he'll misunderstand and think that something has happened to the baby and come rushing over."
So the father-in-law left the following message:
"The clock has arrived, but the pendulum is missing."