A page of a book carries the page number 68 printed in plain block digits. The book is turned upside down, that is, the page is rotated through half a turn. The number then reads as another perfectly ordinary page number. Which number is it?

A half turn does two things at once: it swaps the left-to-right order of the digits, and it turns each digit upside down. Taking them in the new order, the 8 comes first and stays an 8, then the 6 turned upside down becomes a 9, so the reading is 89. The traps: 86 swaps the order but leaves the digits standing, while 98 turns the digits over but keeps them in their old places — each does only half of what a rotation does; 68 assumes nothing changes at all.