A birch and a beech tree grow up side by side in the forest. One day, a sapling grows between them, and each swears that it's not their son.
The beech thinks it's a son-of-a-birch, and the birch thinks it's a son-of-a-beech. To settle it, they enlist the help of a woodpecker, who checks it out.
"You are both wrong," he says when he comes back up. "That's the best piece of ash I've ever had my pecker in."