Two tall trees...

a birch and a beech, are growing in the woods.
A small tree begins to grow between them, and the beech says to the birch
"Is that a son of a beech or a son of a birch?"
The birch says he cannot tell.
Just then a woodpecker lands on the sapling. The birch says,
"Woodpecker, you are a tree expert. Can you tell if that is a son of a beech or a son of a birch?"
The woodpecker takes a taste of the small tree. He replies,
"It is neither a son of a beech nor a son of a birch. It is, however, the best piece of ash I have ever put my pecker in"

... or you could wear a green willow ;-)