"Sudoku," he says, as he sets up a cup of tea and arranges the pain au chocolat, "is a very clever pencil game. You're arranging numbers in a grid, so that you don't repeat a digit within a row or a column, or within one of the smaller grids--oh, it sounds miserable, but it's not really mathematical at all. There's another sort of puzzle, addition, but--"

Erm. He clears his throat and looks sheepish. "I may have discovered a new vice, that's all."