Each of the main chapters outside the prologue and epilogue contains an optional puzzle that lies off one of the corridors between the puzzle chambers. You’ll quickly understand why they’ve been kept apart from the main tests, as most of them are a little out of step with the rest: some are simple, while others are a lot more time-consuming. They are, however, worth taking time out to complete, often expanding upon themes that crop up in dialogue between Tom and Ava: one leads to a Chinese room, another to scattered pages pertaining to Alan Turing’s research. Others function as standalone challenges, including one extended puzzle based on Boolean algebra. Don’t worry: a bit of backtracking aside, the latter isn’t as tricky as it sounds.