Ender spent much of his private study time playing a game called "Free Play."  He had worked his way through the obstales to the part of the game where he had to choose one of the two Giant's drinks.  If he got the wrong choice his avatar was killed off in the game.  If Ender could choose correctly he would be admitted to a virtual "fairyland." 

Ender was frustrated and could not seem to get past the Giant's Drink.  He was beginning to believe there was no way to win and that the game was set up to be unbeatable.

At the beginning of Chapter six in "Ender's Game," Graff is being questioned by a superior officer.  He tells Graff that Ender is stuck on the Giant's Drink in the mind game.  The officer asks Graff if Ender is "suicidal." Graff states:

"Is the boy suicidal? You never mentioned it."

"Everybody gets the Giant sometime."

"But Ender won't leave it alone.  Like Pinual.'

"Everybody looks like Pinual at one time or another.  But he's the only one who killed himself.  I don't think it had anything to do with the Giant's Drink."

This discourse lets the reader know that some student's have become obsessed and depressed over not being able to get through this part of the game.  One student even killed himself.