Far, far below Yggdrasil's roots lived a dragon called Níðhöggr. He was old and grey and grumpy, and he chewed on the tree's roots.
Every day.
Gnaw, gnaw, gnaw.
"Why do you chew?" asked a little serpent who lived nearby.
"Because it needs doing," said Níðhöggr.
"But it doesn't taste good?"
"No."
"And it never runs out?"
"No."
"And you never finish?"
"No."
The little serpent thought about this. "Then why do you do it?"
Níðhöggr stopped chewing. He looked at the serpent with his old, yellow eyes. "Have you seen the tree?" he asked.
"Yes," said the serpent. "It's enormous."
"It's enormous," said Níðhöggr. "And it holds up everything. All the worlds, all the mountains, all the seas, everyone who lives up there. And do you know what happens to things that carry too much?"
"They break?"
"They break. Unless someone takes away a little at a time. Unless someone chews away the old to make room for the new."
The little serpent thought about that. "So you're helping the tree?"
Níðhöggr grumbled something and started chewing again. "I never said that," he said between chews. "I don't like the tree. I've never liked it. It takes up all the room and all the sunlight and everyone talks about it as if it were so wonderful."
"But you're helping it anyway?"
Níðhöggr chewed harder. "Go away," he said.
The little serpent slithered off. And Níðhöggr went on chewing, and if he smiled a little then it did not show in the dark.