Hogfather - See

Overhead, heedless of the new and somewhat angry life that was even now dusting itself off, the sledge soared onwards through time and space.

I'M FINDING THE BEARD A BIT OF A TRIAL, said Death.

'Why've you got to have the beard?' said the voice from among the sacks. 'I thought you said people see what they expect to see.'

CHILDREN DON'T. TOO OFTEN THEY SEE WHAT'S THERE.

'Well, at least it's keeping you in the right frame of mind, master. In character, sort of thing.'

BUT GOING DOWN THE CHIMNEY? WHERE'S THE SENSE IN THAT? I CAN JUST WALK THROUGH THE WALLS.

'Walking through the walls is not right, neither,' said the voice from the sacks.

IT WORKS FOR ME.

'It's got to be chimneys. Same as the beard, really.’

***

Hogfather - p67