‘It's gettin' late and we've got lots ter do tomorrow,’ said Hagrid loudly. ‘Gotta get up ter town, get all yer books an' that.’
He took off his thick black coat and threw it to Harry.
‘You can kip under that,’ he said. ‘Don't mind if it wriggles a bit, I think I still got a couple of dormice in one o' the pockets’
That's from Harry Potter and the Philosopher's Stone chapter 4, when Hagrid has met Harry in the hut-on-the-rock, after he's told everything necessary to Harry.
What's the story about the mice? Why did Hagrid keep mice with him in his coat pockets?