It soon becomes evident that Professor Hawking and Mr Mlodinow regard a philosophical problem as something you knock off over a quick cup of tea after you have run out of Sudoku puzzles.

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The best way to prevent dementia is to keep the mind active, whether it's by reading books, doing crossword puzzles, playing sudoku, or participating in other creative endeavors, says Kennedy.

CNN: Study: Ginkgo doesn't cut dementia risk