one for sorrow

Can anybody explain this nursery rhyme to me?
I can understand "one for sorrow" and "two for joy".
But why ...

"Three for a girl,
Four for a boy,
Five for silver,
Six for gold,
Seven for a secret
Never to be told" ?

These are magpies - birds which can be rare in England (Not at the moment). Generally they are seen alone or as a pair, so the larger the number together, the more unusual it is.

Magpies have a fearsome reputation, and it is very important to give them a proper salutation. For more about this see: Magpie superstitions