In Paulo Coelho's The Alchemist, the alchemist and the boy are captured by one of the desert tribes that is at war. The alchemist gives the boy's gold to the chief and then tells the man that Santiago is an alchemist and is so powerful that he can turn himself into the wind and destroy them all. The chief is curious and demands to see if Santiago can do so. The alchemist notes that the boy must have three days to prepare—that he will only turn himself into the wind to show them what he is capable of doing. Santiago is very much afraid, knowing he cannot become the wind.
This is why the alchemist puts Santiago in such a terrifying situation. The boy is learning the ways of the desert and the Universal Language, and feels that he is approaching his Personal Legend. However, up until this point in the story, it has been greatly academic—Santiago has been able to reason his way between the different phases that he must pass through in order to realize his Personal Legend, but he has never been required to put what he knows into practice—which would change the way Santiago looks at himself, the world, and his place in the world.
"Don't give into your fears," said the alchemist, in a strangely gentle voice. "If you do, you won't be able to talk to your heart."
"But I have no idea how to turn myself into the wind."
"If a person is living out his Personal Legend, he knows everything he needs to know. There is only one thing that makes a dream impossible to achieve: the fear of failure."
The alchemist "pushes the envelope" by putting Santiago into a situation where he must do all that he is capable of (especially things he believes he is not capable of) in order to realize his own potential.
It is for this reason that the alchemist puts Santiago to this test: so Santiago can realize what he is truly capable of achieving.