Good Morning Mr. Magpie
There are hundreds of superstitions about magpies, but many of them derive from the fact that magpies are believed to be the Devil in disguise. If you see a solitary magpie, it's the Devil, and you should say "Good morning Mr. Magpie, how's your wife today?" to show the Devil that you've identified him and he can't cause you any trouble.
And of course, nobody should forget the rhyme: "One for Sorrow, Two for Joy, Three for a Girl and Four for a Boy, Five for Silver, Six for Gold and Seven for a Secret, never to be told".