Why do Christmas trees have a fairy on top?

It was three days before Chiristmas, and Santa was having a BAD time:
Half the elves were down with a flu bug.
Mrs Claus was on a work-to-rule - no mince pies or sex until the New Year.
"Prancer" wasn't prancing - he had a hoof infection, and to cap it all he couldn't source a replacement ( Toyota ) bulb for Rudolf's nose......
There came a knock on the door, and when he opened it he saw a small fairy struggling to hold up a large fir tree. "what the hell is that"? asked Santa.
"It's called a Christmas tree" said the fairy, "in years to come, everyone will have one of these, beautifully decorated with streamers & lights. We thought it would be fitting if you had the very first one - perhaps you could decorate it yourself? Now where would you like me to put it"?

Good One :}