The Giant's Drink

"One is poison and one is not," said the Giant.  "Guess right and I'll take you into Fairyland."

                                                                                     -The Giant in Ender's Free Play game

This is an interesting scenario that is referenced several times later in the book.  The children at Battle School are given a game on their desks called Free Play that builds a sort of virtual reality world that they can explore.  The computer is sophisticated enough that it can create many different scenarios personalized the player and created to challenge them.  One of these scenarios is The Giant's Drink.  In this scenario, you come upon a giant who promises to take you to Fairyland if you pick the right drink out of two.  One is poison, and will kill you in one of numerous awful ways.  It is mentioned several times by the two mysterious voices that the Giant's Drink is an impossible game to win, and they are worried that Ender won't give up trying to beat it.

This is where things turn Star Trek, in Kobayashi Maru style.  Ender is fed up with the unbeatableness of the scenario, and he attacks the Giant and destroys his eye, killing the Giant for the first time in the history of the game.  Ender is the only student who has ever thought of a way to beat this game.