Alice had seated herself on the bank of a little brook, with the great dish on her knees, and was sawing away diligently with the knife. 'It's very provoking!' she said, in reply to the Lion (she was getting quite used to being called 'the Monster'). 'I've cut several slices already, but they always join on again!'
'You don't know how to manage Looking-glass cakes,' the Unicorn remarked. 'Hand it round first, and cut it afterwards.'
This sounded nonsense, but Alice very obediently got up, and carried the dish round, and the cake divided itself into three pieces as she did so. 'NOW cut it up,' said the Lion, as she returned to her place with the empty dish.

Through the Looking Glass (And What Alice Found There), Chapter 7 - The Lion and the Unicorn