When you are young you can assume that old people are ignorant about all the clever things you can do as a young person, and have no idea what its like to be young. This saying is a reminder that they used to be as young as you, and were up to the kind of clever things you think you invented last week. Getting the contents out of an egg without cracking the shell is one of those things.

Of course it comes from a time when there weren't so many novelties around, like iPods and electronic games and skateboards. But the truth is still there - your elders have learnt just as many tricks of the trade as you have, and any wise young person is going to appreciate that, and not try to patronise them.